I struggle at 3 sometimes, 4 is about my limit. There's a huge gulf between "I know how the pieces move", "I can find an easy mate in the first 30 moves" and "I can beat Stockfish level 5".
I'm going to repeat this for everyone : Castling is vital to becoming a decent player. Learn how to do it, when to do it and fight like hell to maintain your ability to do it. Don't go moving your king out of check when you can sacrifice a piece instead. Castling (usually) moves your king to a safer area and mobilizes a rook in the same move. It's crazy to neglect it.