Never hit your pup. It does him no good, and he has no idea why you did it. I know you already know this because that's why you are here asking for help. I am glad you came here.
OK, deep breath.
You need to be aware of the times of day you KNOW your pup needs to relieve himself. After eating and after play and exercise is a common time. If you are pee pad training, you need to confine him to the pad and use key words like "go potty" when you want him to do his business. If he doesn't do it right then, you need to keep an eye on him until you see the "signs" that he is going to go. You know, sniffing around, circling, etc. Put him on the pad, and say "go potty". And when he does, you need to praise praise praise. I treat as well, but some folks don't. Praise can be enough.
Expect accidents. A four month old will have accidents. If your pup is like most of ours, you need to expect accidents for maybe a lifetime! But you can train them down to very very few!! Patience patience patience! And if you find that you need to yell and hit, roll up a newspaper, go in the bathroom, shut the door, look in the mirror and hit yourself over the head while chanting,"I should have been watching him"!!
You may also want to consider crate training. You can do a search on that and find tons of info!
Good luck, and give that puppy big loves, OK?
And welcome to YT! You did good by coming here!