Hey- I study Biology, Chemistry and Psychology (I want to study medicine).
I would NOT recommend taking four A-levels (loads of people dropped one).
So, I find chemistry ridiculously difficult (so many people are getting like 30% in their topic tests- it's crazy).
I LOVE biology (then again I always have)- people are getting around the 50s in their tests (but apparently that's good for this stage). There are some difficult aspects, but others are easy and interesting.
Psychology is hands down my favourite subject- but it's definitely not for everyone. Its literally an essay based science- there's a lot of experiments you need to learn as well as research methods etc, and you have to be able to apply the experiment studies to questions from the topic. Its not necessarily difficult- but there's loads of content.