If you're interested in pursuing psychology at uni then, do psychology, biology and english literature as a minimum. Why? Because you need psychology a level to have a basic understanding, biology because at uni you will learn about things like the brain so it's helpful to understand the basic biochemistry of the body at a level, rather than trying to learn them at uni. English literature because like psychology, there's a lot of interpretation to be done, plus, both subjects requires a lot of written work.
I personally didn't do 5 a levels, i did 4AS, and 3A2s, and although i'm not in medicine, i'm in dentistry, so..... no point lol.
As for a fourth a level, chemistry or german? I would do german because in the future, you can have german patients or go to germany to work, and language is a real plus to have on your CV nowadays in this competitive job market.