A bit late, but for anyone else wondering this might be helpful. I took 4 HLs and would credit this for my weirdly low offer for Cambridge (the average offer for my course was 43, but my offer was 41), as well as for getting me into 4 of the top 10 unis in the world (American unis understand IB and so are much more impressed by 4 HLs) so definitely worth it if you're applying America or Oxbridge. It's manageable, as long as you have good time management (something learned the hard way!) and I still got 45 in the IB (admittedly after a remark). However, I'd say choose your 4th HL carefully! My first 3 that I was always going to keep were Maths, Chemistry and Physics, my fourth being Philosophy. The good thing about philosophy is that there is very little difference between SL and HL, and so it was completely manageable as a fourth HL. The same goes for languages (including English) and Econ as far as I can tell. Something like Geography or Art however are far less manageable as they take up a lot of time. As long as you choose carefully and enjoy working hard, 4 HLs are fine!