I'd go for psychology over RE, but that's my personal bias after 5 years of sitting bored in a cold, dark RE classroom. You've already got the choices you need for a sciencey career, so it's up to you and what you think you'll prefer.
Not doing physics or further maths rules you out from taking some science based degrees which is something you should consider. Outside of that though, your fourth A-level can be anything you want.
I'd go for psychology over RE, but that's my personal bias after 5 years of sitting bored in a cold, dark RE classroom. You've already got the choices you need for a sciencey career, so it's up to you and what you think you'll prefer.
If I were you, I'd probably go for Psychology since it's more science based and it seems more interesting (to me personally). On the other hand, R.E may show that you're an all rounder and would give you a break from all the science-y subjects. Just go for the subject you think you'll like more or feel more passionate about. Good luck!