Yes, there is an expiration... IF you are trying to resell the seat. 5 years from date of manufacture is the limit by law. NO, you do not need to replace your seat. If it has some serious wear and tear, sure, but if it hasn't been in an accident or had some sort of structural problem, do you really think it is unsafe? Those things will take forever to degrade in the landfills, I don't think you have anything to worry about after only 4 years of use.
We used our seats for 2 kids and then passed them on to friends. I have never known anyone to purchase a new seat due to expiration, it really just affects you trying to resell it.
You could contact the manufacturer if you want to feel more secure about the decision, but I'd get them to PINPOINT what they think the problem is with the chair before you get rid of it. I think it is a shame so many seats that are perfectly fine are in the landfills.
Take care,
Liz
p.s. My sisters used their seats for 9 years and cycled through 3 kids each and the seats were in good enough shape to pass on to friends. They are basically large hunks of plastic, as long as the straps are in good shape, you should be fine. They are built really sturdy.