maybe your dairy is different than ours-- but by the time they are of that age, if they don't milk.. they're already gone... heck, come to think of it-- by the time they reach their 2nd month into lactation as a 2 year old.. and they dont' milk-- they're gone....
It's going to be pretty hard to find this type of cow-- no reason I can find, that a cow would make it to 4 years old, and then bam.. stop milking...
I hate to tell you, but whoever is telling you this.. is screwing you over (or planning on it)
8 year old dairy cow, is pretty uncommon... By this time, they're udders are shot- and would be unable to raise a calf on their own, without TONS of supervision during the first 3 days. Not only that, but most have been culled ruthlessly for repro..
Do you realize, the turnover rates for dairys? and that repro is the number 1 culling factor by 80%?