As you identify, you were a 4+ year user, past tense. That is, you already used the service and you have therefore paid the fee to use that service. You are not entitled to proration sir just because you have used the service in the past.
However, if you paid for a year and are only partially into the service agreement period, then yes I agree you should get some proration in return. Aside from that, seems like you have paid and used the service provided.
He does not indicate he is a past user. What he said was "As a 4+ year user...", then goes on to say that he continues to use his .mac email address which would seem to indicate that he is in fact a current MobileMe customer.
Assuming all of what I just wrote is true, then I think he is correct. IF the service is switched to free then FOW (and myself and other current MobileMe subcribers), should be prorated for whatever length of service is left on our contract. I hardly find that unfair.