How can I stop my friend from using my account on his PS4?

Note that it's never a good idea to do account sharing as it can lead to all sorts of grief for the affected people if there's ever a disagreement.

Having said that here's some ways to resolve this:

1. Talk to them

If it's a simple misunderstanding (them using your account for online play, messing up your stats etc), then simply asking them not to do that is an option. Be calm, but firm: you dislike that they are doing X, and you wish that they would stop.

If they truly are your friend, it's likely they will understand.

2. Reset your password

If the diplomatic option doesn't work, then it's time to change your password (and not tell them the new one). Log into your email you use for PSN, then follow the prompts on Sony's page to change your password:

Change Password: Sony Entertainment

You will be sent an email with a password reset link in it. Click that and change your password (and don't just add a '1' or something to the end, try and make it very unique).