I give out a simple 10% discount code with every order. That's more or less the reward, really. However, if they come back to me often I'd add in an extra free gift every now and then, or offer them a bigger discount.
If they've bought from you 4 times then I'd say that a free gift or a large discount was a good idea, but the free gift would have to be a good one. In this situation I'd add in one of my cheaper products rather than the small items I usually give out as free gifts with orders of 2 or more items, or that go out with every order of any size during December. It's always nice to get a free gift in December!