Is it possible to do an 8 tooth bridge with 4 supporting roots/teeth?

It is difficult to fully answer your question without xrays and photos. If the teeth are strong enough and supportive enough, it seems that you could do a bridge. Since you have 3 missing teeth,#9,10,11 between the 2 anchor teeth on the left, I would go further back on the bridge to include one more tooth #13 for more support. #11 takes a lot of force(it has the longest root in the mouth) so you want it well supported. The fact that you have had root canals makes the teeth weaker and more brittle. I would be concerned about the longevity of a bridge this extensive. If something goes wrong with one tooth, you basically have to redo the entire bridge. Anytime you can split it up into sections, the better from that standpoint. Have you looked into implants especially on that left side. That would give you the option then of a bridge from #4-8, then an implant in the #9 and #11 position maybe and a bridge between those 2 implants. that would be much stronger.