IMO, as long as the customer is informed of the backlog and understands the system/sequence in which the business does the work on the customers item and still wants the job done, he/she has no complaint whatsoever.
If I go into your shop and you tell me there is at least a four year wait and I agree to have you do the work, I should expect to wait four years.
It would be when a customer brings an item in for work and is given an approximate time for services rendered and the work is not done on about the given time, or a reasonable time thereafter, that would make me upset...understanding that there is always cases in which things can go wrong and the original given dates can't be met. Then a simple phone call or some kind of communication by the business should be given to the customer.