How much hop is correctable, and beyond which point should a rim be replaced?
The answer depends on the reason for the "hop" and your desire for perfection. 

A wheel which is out of round due to incorrect spoke tension, rather than impact damage to the rim, will have no trouble being repaired, even with far greater than 3-4mm of deviation by detensioning all the spokes, and re-tensioning them evenly, essentially re-building the wheel.

A wheel which is out of round due to impact can be made round with that much deviation, or more, but it requires having the spoke tension be unbalanced. If the spoke tension on a wheel is unbalanced by more than a few percentage points, then the wheel will not remain true and round. It may, however, remain safe to ride, and only require more maintenance than normal to keep it straight and round, or it may not be safe at all. 

That is something only a skilled mechanic or wheel builder can tell you, and only after re-tensioning the wheel.

Since it is a factory built wheel, I assume either that you tried to true and tension it yourself, or that there is impact damage, because it should be round from the factory.

I hope that helps.