I’m currently writing a game using v4 and I seem to be doing well. My play testing isn’t crashing thus far.
Only Apple knows this. Currently OpenGL is deprecated. Apple seems to have a longer track record of “deprecating but not immediately removing”.
I have two older apps that someday I need to update, but for now they work great still. They are using API calls that Apple deprecated in 2012 and in some cases 2006.
v4 is a bit slower than v3 in rendering. Can your game handle it? I don’t think you will even notice from the games I have moved from v3 to v4. They feel exactly the same.
Make sure you play test a lot.