I'm not gonna lie, the best thing to do is just rest over the holidays. If you want something specific to do, look for supercurricular activities which can help your personal statement.
I'm at the end of Y1 doing Maths, Further Maths, Chemistry and Biology, and can tell you that it's just better to come.
That being said, I would make sure you're organised. Make sure you have a day folder which contains everything you'd need daily for all your subjects, and 4 larger folders to keep at home. Also, try and figure out how you best revise, so you're ready to keep stuff in your head.
Apart from that, just prepare mentally for 4 A Levels.