Hi! I would think about what degree/career you have in mind before making this decision. The thing is, a fourth A Level isn't likely to give you extra points to get into uni because unis will only look at your 3 best grades when offering you a place. Although an EPQ may not lower your grade requirements for some universities, if you do it about a topic linked to your degree, it'll be an invaluable thing to write about on your personal statement and it might prompt conversation in interviews (if you're thinking about medicine or going to Oxbridge). It's seen as a 'super curricular' by unis if it's linked to the course you're applying to, which will be important in showing your interests in the subject, your passion, time management, and that you do things to further your education outside of your A Level curriculum.
I think the best thing to do is to think about what course you might want to apply to if you want to go to uni, think about whether you want your EPQ to be in the same field of study as your university course, and think about what you might want to know more about. I want to go to law school but when I did my initial ideas, I had ideas that linked to literature, PPE, history and law before I finally settled on a law topic.
Ultimately, I can't say which one is better because I don't study psychology, but I do definitely think that 3 A Levels and an EPQ would be more manageable than 4 A Levels.