Season 5

Premise
Known plans for  of Sanctuary included working from the new Underground Sanctuary and the long awaited for return of fan-favorite series regular character Ashley Magnus. Amanda Tapping has stated in several interviews and at convention panels that Magnus had found a way to save her daughter during her 113 year trip back through time without interfering with the course of the natural timeline. Amanda had conferred with Emilie about this, to which she was on board to reprise her character. Unfortunately the show was cancelled before this plan could be filmed and shown in-universe.

How Magnus could have saved Ashley
The set up for this already exists in-universe as seen in when Magnus is searching for a way Ashley could have potentially survived her supposedly fatal teleport. In her search, Magnus's analysis of the computer recordings from the EM field that night show that it had detected an unexplained weakening in the EM shield at the exact moment Ashley teleported at 18:29 Hours; possibly letting her pass through the barrier. Neither Will nor Magnus could figure out the real reason for why it had occurred outside of throwing around personal theories. None of Magnus's tests showed that Ashley landed just outside of the Sanctuary or anywhere inside the building, nor still potentially stuck in the EM shield's buffer system. (All of this evidence just simply supports that Ashley isn't physically 'there' at the Old City Sanctuary anymore, not necessarily that she's also been disintegrated.)

When Magnus went back in time, she absolutely knew that she could not directly interfere or stop terrible things from happening; having to let things play out, as any noticeable change could derail the course of natural history. Being that Future-Helen was able to secretively create the Underground Sanctuary by cleverly weaving her way through time to build it, it is possible that Future-Helen knowing about Ashley's teleport had intentionally interfered with the EM shield to create a tiny 'hole', snatching Ashley's energy signal mid-teleport, to redirect her to land at the new sanctuary; keeping her hidden there for the next three years until after the end of Season 4 where she would have finally been revealed on-screen in Season 5.

This theory is highly plausible to have occurred, as Ashley being alive for all of this time wouldn't interfere with history considering that she has still been physically removed from the known events of the Sanctuary timeline by having her live underground instead of up on the surface; most likely spending her time there helping her mother operate the new sanctuary.

Cancellation
Several factors had culminated into the ultimate decision by Syfy to officially cancel the show, preventing Sanctuary from having a fifth season.

Independently financed
Sanctuary is a completely privately financed TV series; not funded by large production companies like the mass amounts of other regular TV shows. During the time the web series was transitioning into becoming a TV series, due to running out of money from not making enough revenue back from viewer webisode purchases as the show was heavily pirated across the internet, Sanctuary creators had to sell the mass majority (if not all) of the show's rights to their private financier "The Beedie Group" (an industrial development company - who were fans of the science fiction genre) in order for them to continue to invest heavily in the show as a single TV episode of Sanctuary typically cost $2 million in production expenditures; they had already invested at least $5 million into the webisodes and a total $26 million per 13 episode season / $40 million for Season 3, gap financing the entire project and by the end of the show had finally started to see some returns.

Parent company
Syfy's parent company NBC Universal had been recently acquired by Comcast on January 28th, 2011. Because of recent changes in ownership and other ongoing trade-dealings etc., by the time early-2012 rolled around for the usual television show renewal process and announcements, Syfy was put on hold; not being able to green-light series productions. Syfy itself wanted to renew Sanctuary for a fifth season consisting of six episodes.
 * Note: Around this time, Networks had started a new practice to give popular shows 6-episode final seasons in order to wrap up storylines; so as to not end on a cliffhanger that would frustrate audiences.

Even though all other international markets had wanted to see Sanctuary continue and had agreed on a fifth season renewal, The Beedie Group would not move forward without a confirmation of backing support from the major US market.

Lease renewal
At the same time Sanctuary was stalled on renewal from Syfy, the lease for the building the show used to film in had come up for renewal as well. To continue to use the building, The Beedie Group would have to sign a new contract to rent the property for a span of 3 more years.

Decision from The Beedie Group
Due to the uncertainly of renewal caused by delayed response from Syfy, eventually hearing back (too late) from Syfy about having only a 6-episode long Season 5, and having to sign a 3-year lease to rent the property, The Beedie Group made the financial decision to stem their losses by altogether pulling their funding and not resigning the lease; effectively putting an end to the production of the tv series.
 * Note: Sanctuary was a passion project for The Beedie Group and not the company's primary focus nor did the show have a large sum in ROI; waning the group's interest in providing continued finance. Many cast and crew penny-pinched the entire run of the series, volunteering portions of their pay to go back into the show as they valued the experience to work on the project more so than their monetary compensation for their time and efforts. A notable example of this was Amanda Tapping who took a small pay as an actor and donated her added pay for being an Executive Producer. When the decision to not renew the lease occurred, the cast and crew were suddenly left on short notice with a mad-rush of 3 days to vacate and clear out all of their belongings and show items, etc. from the entire building and offices. Many of the show items / props that were saved were donated to the charity organization "Sanctuary For Kids" and auctioned off in order to raise funds.

Announcement from Syfy
Now that the show had lost their location space and their major financial backer(s), Syfy in turn had to announce the cancellation of the series on May 21st, 2012.

The future of Sanctuary
Due to The Beedie Group owning the rights to the Sanctuary IP, they control the release and dissemination of Sanctuary related materials. Even though Damian Kindler, Martin Wood, and Amanda Tapping had wanted to officially release original music created for the show by Andrew Lockington, they couldn't move forward with the plan without approval. Any future seasons, web-series, movies, spin-offs, reboots, comics, books, apps, video games, etc. with the Sanctuary IP are essentially dead-in-the-water unless The Beedie Group are willing to either sell the rights or decide to get re-involved with the project.

Updates

 * Update: Continued talks of potentially still trying to release things such as the music or create more content had been "started" as late as just before September 23rd, 2018 (upload date of video source).
 * Update: As of Amanda Tapping's interview with Suanne Braun on April 4th, 2020, the IP / rights to create new Sanctuary content and sell existing episodes have been bought by an unnamed someone.
 * According to a report by Deadline.com (further reported on by BloodyDisgusting.com on March 10th, 2020), “Executive producer Martin Wood and All Canadian Entertainment have inked a deal with original rights holders Beedie to start development on new installments.”
 * Update: June 30th, 2021 - A GateWorld.net article discusses a recent Deadline.com article. A Sanctuary reboot/miniseries is still in a development process.
 * June 30th, 2021 - Excerpts from a new Deadline.com article about Amy Hartwick becoming president of MPCA: "Among the first projects she’s taking on is a reboot of Damian Kindler’s 2007 sci-fi series Sanctuary, about people and creatures with extraordinary powers, which ran for four seasons on Syfy. ... Sanctuary and Just My Luck were projects on MPCA’s slate that Hartwick identified when she started at the company last month and began pursuing. She also has been acquiring IP and developing original ideas that fit indie MPCA’s mandate as a supplier of four-quadrant content."