When we bought our house about 12 years ago I took the floors down to bare wood and sealed, stained and finished them with Gym Seal. The main selling point for me was the idea that I could sand it a bit and refinish as needed, as opposed the a lot of other product that would require sanding back down to bare wood if they ever needed a redo.

We got a lot of use out of the floor (with some judicious use of runners and doormats) but it needs some help now. So I go to my big box store and find that Gym Seal is no longer being made! What is a good replacement now? (And please don't say that I've got to strip it down completely and start over...)

A few facts:
The floor is about 110 year-old pine (we think from the southern US, old-growth). 12 ft lengths.
We used 4 coats of gloss Gym Seal.
Always mopped with soapy water - no waxing.

Any suggestions?

u can get gym finishes..most of them are poly now,,,but some still manufacture the old varnish type finishes..different resin base then poly.

keeping it simple..you can use poly now...but i would say your at the end of your rope here..already 4 coats , now adding 5th..
floow will be susceptible to scratches and denting now....go for it..recoat...
wipe floor down with mineral spirits first to remove soap contaminates..