I agree with the idea that he's been fine for 4 years, but you asked what else can be done:
Sand is ok, not great, the idea is that the gecko may eat too much sand either by accident when eating or on purpose to get more calcium. Sand is a huge no for younger geckos, but adults that can tolerate do fine. I prefer to keep all my geckos on papertowels, much easier and cheaper to clean for me. But if you have no problems at 4 years, most likely you won't get any.
The big problems with geckos are A) substrate B) heating C) vitamins and minerals D) right kind/size/amount of prey.
Since you've had the guy 4 years and haven't noticed any problems, most likely you are covering all these areas. The only other thing you may not notice not having other geckos is calcium deficiency. But if you are feeding dust, gutloaded prey of some sort it is not an issue. If you notice anything odd about the legs or feet of the little guy, have it checked out.
Oh, and he's most likely shed about once a month or so. Watch for pale coloration or a ghostlike appearance. These are signs he's about to shed as the top layer of skin separates from the new layer. The next day his colors will be the brightest. It is a good idea to check his tail and toes to make sure he shed completely. Skin buildup there can cause problems and loss of claws.
If you'd like to check on your temps and setup in detail, I always recommend "the leopard gecko manual" it is less than $10 online or in bookstores. It is never too late to make everything perfect, but it sounds like you must being doing ok. Good luck!
-----
0.1 Albino Leo Gecko
0.1 Crested Gecko
1.0 Dear Boyfriend
Departed: Harvey and Spock
I wouldn't worry too much.