Jon; No crystal ball nor history to predict. My own personal gut instinct is that there will be no noticeable resale value. Age, condition of the unit, and hours will play a greater role.
Don't worry about getting $500 back in 15 years. Even with moderate inflation, the $500 will probably be equivalent to looking at a $100 difference now-a-days. (Guess… didn't bother running actual numbers).
Given the way HP has increased over the years, within 10 to 15 years 4 HP will be viewed as a nit. In the future, they will probably be selling rider mowers with 50 HP (or, at least, that's what the manufactures will claim /w3tcompact/icons/smile.gif).
Roy