If you’ve got a four year old definitely get a full fat switch. Makes co op gaming together much easier if you do it together on a big screen
Depends on the game. My nephews managed Mario kart fine at that sort of age
With kids? Mariokart! And Mario Odyssey. Otherwise depends what you like. Lots of very highly rated rpgs if that’s your thing - skyrim, witcher 3, dark souls if you like a challenge. Fire emblem if you like turn based combat.
Decent case and screen protector