Psychology doesn't really complement Chemistry... Maybe the other way round, but uhh, yeah there's approximately 0% psychology involved with studying chemistry. Anywho, it doesn't matter, your fourth subject need not be relevant. Physics technically would be the best option (major part of some chemistry), but you can live without it.
Lots of people also like psychology so swings and roundabouts. The general gist seems to be: moderately interesting for most people, lots to remember (case studies etc.), not really essay based for AS. There will always be people who dislike stuff, i'm sure plenty people will happily tell you how chemistry was the worst choice they ever made
so don't worry about people putting you off too much
Few comments there from a few people, literally just stumbled on an old thread. I thought there might have been a general chat/discussion thread on the Psychology forum to help you, but it doesn't look like so much. There are a few other people asking the same question though along the way so just some example responses there.
I remember doing part of biology AS which had an insight into behavioural response, probably bordering towards psychology side of things, was quite interesting. That was OCR board.