Honestly imo both games are pretty good.
I have no idea what either are like on the switch but as someone who's played both on PC, I'd honestly say choose which ever you'd think you'd enjoy the most. Note though, the switch version of 3 isn't the new remaster version, it's just the original game. The switch was cheated heavily on it.
A couple of the DLCs in 4 are pretty good, but they are very adult lmao. Both games certainly do live up to their age rating xD
Apparently the switch version of 4 also includes Gat out of hell, but I don't really reccomend playing it. I just about 100%'ed it in 7-8h without even trying and it wasn't very interesting at all. Might as well just stick with the main game if you get that one.
EDIT: Oh yeah, 4 is still GTA style btw. It's just that you have superhero powers ontop, which honestly just makes it more fun imo since you can run and fly around the place and just smash ALL the things.