The spring assembly didn't seem to change much for the Glock 26. (Perhaps I'm wrong?)
No, you are right. For the GLOCK, 26, that is. Since the smaller Glocks like that basically already had dual recoil springs anyways, so the gen 4 isn't really any difference for the Glock 26.
Now a Glock 19 say gen 3 vs gen 4 has more differences - the gen 4 has the dual recoil springs while the Glock 19 gen 3 does not. But that's for the Glock 19, you don't need to worry about that since you're talking about the Glock 26.
So for a Glock 26, get the gen 4. It's the same stuff internally but had a few improvements like grip and mag release. If same price get the gen 4