Congratulations on your four years! My fiance and I have been together for almost three, and we're really looking forward to tying the knot when we make it to the US.
Every year the most meaningful present I receive is a photo book (shutterfly.com). She picks out the photos, and adds personal quotes and other touches to each page. Then the company prints and binds it into a hard cover book. I think they're only about $25 dollars each, and maybe you could do one every four years or so to document your progress as a couple.
Alternatively you could make her a special homemade dinner with music, candles, etc. You can't go wrong with something like that. You said your wife already has a ton of stuff, so don't add to the clutter. But if you do want to buy something, try a meaningful pendant she can add to a necklace she owns already. Just don't make it those tacky charm beads.
