I was told by someone who reviews applications at my school that it is better to pick your best 3 letters because this will give you a better "average" LOR score. Unless you think all 4 are equally good, he suggested going with 3. He also said that the more letters you have, the more likely people are to skim them rather than read them through.
That said, if each letter contributes new and important information rather than just another person saying how awesome you are (or if each is from someone very important with a lot of influence in your chosen field, AND each says great things), it's probably a good idea to send 4.
This was the advice I was given, and it made sense to me, so I only sent 3 letters to any given program (even though I have 6). Maybe others have different ideas.