4 years is a pretty good payback time, it's a pretty obvious investment if you can afford it, and it's pretty affordable (with the added bonus that if you're renting, you can take it with you). Solar panels tend to last a pretty long time, really the only worry is whether the manufacturer has cheaped out on the electronics (cleaning is literally going to be wiping it with a sponge once a year or so).

(Insulating your house or getting double glazing, for example, takes a lot longer to pay itself back, and these are pretty popular home improvements despite that and the higher outlay)