Just happened to drop into the forum and thought I'd throw this out there; I'm not a mod nor a breeder so we'll have to see what others might offer you, but... I can't help but think that there might be a bit of telling prospective buyers what they want to hear, and in a way that is not totally disingenuous or misleading, depending on what sort of results or outcomes an average hobbyist might be willing to accept. (I think it can also be a way of trying not to immediately discourage people who have some legitimate interest but are dubious about all the WCs people like us on SD talk about!)
In other words, I think a breeder might well imagine that someone who will otherwise watch his fish carefully, keep them in a large enough tank at the right temperature, etc., can probably keep 3-4" discus alive and hopefully without health problems for some period of time (maybe months or more) if they do weekly WCs.
The thing is, keeping them alive is a world away from seeing them really thriving. Part of it is growth rate and proportion, color and the like, but part of it is also behavior. Trust me on this one, it's no fun having one fish that stops eating, darkens and huddles in the corner, or starts dashing and smashing against the glass because it's bugged by a pathogen that was able to reproduce in a tank enough to wear down its immune system. (And if you've made it to SD, you can see the difference between the "photo winners of the month" and the pics in the ER threads...) Maybe there's a little bit of "middle ground" between these extremes but the hobby is so much more fun when they're "realizing their potential" rather than "hanging in there."
That said, I think the general consensus on here is that adult fish around 5-6" will do alright with a lighter WC regime--and maybe that isn't too far off from the 4" inch range you're talking about--but you really have to watch them and make sure what you're doing is or isn't working. And if you check out the people on here who are currently breeding their fish (check "new posts" and you'll see the names of some folks with fry right now, and then follow those people's other threads on grow-out and such), you'll see what they do to actually keep their breeders and raise their fry. (I've been learning a lot from them, and among other things, they change a lot of water.) So, I'm not sure your starting premise about breeders vs. hobbyists is really a good one. (Or that the breeders you might have talked with are recommending something that they actually do themselves.)
Anyway, I just saw your other thread and it looks like you're getting yourself well set-up so you can do what you need to do with the fish you're getting in. Good luck with them and hope you come to enjoy some of the maintenance... it's not bad once you see how much it helps them thrive! Best of luck with the new group.