Will Gen 4 Mag followers work in a Gen 3 mag? . Followers differ and are they compatible. Any and all appreciated. 15 rounders in question.

It appears they are all the same, check this link
https://store.teamglock.com/glock-magazine-follower-10mm-all.html

I seen that. Notice in description 10 round capacity. Is that a game changer for 15 rounders?. Thanks in advance.

It should not matter 10 or 15 round mags, the follower is the same.  The older guns use a slightly different follower but these are the newer redefined as used on all.

I'll give it a shot. Thanks for input. Let ya know.