Someone can present you a bill--and if you don't pay it, sue you--any time before  the "statute of limitations", or time within which to sue, expires. (Technically, they can bill you after the statute runs out, but since they can't sue, they can't enforce the bill--it would be 100% voluntary whether you pay.) In your state, the appropriate statute of limitations (contract: the claim arises out of the agreement to provide certain services for pay) is 5 years, so if this happened 4 1/2 years ago, they are still in time to take legal action if they want.