Apple is pretty obnoxious in this area, so I would definitely wait until at the very least they offer free upgrade vouchers in the box if their new HW doesn't ship with X.4.

Frankly I think they should just go with 'X' (the letter, not the Roman numeral) with a version number, instead of prolonging this 10.x.y stuff. I mean, each of these 'sub versions' rank (and are priced) as version releases due to new apps, features and fframework/APIs....