Well, first of all congratulations. 4 years is a long time.
You don't have to get him something expensive, I'm sure he will appreciate a homemade gift (perhaps a collage of photos of the two of you). Another suggestion is a short poem, a video game (if he likes video games that is). Last thing that I can think of is on pieces of paper, write reasons why you love him and then putting those pieces of paper in a jar. Cheesy I know, but I think it's cute haha.
