If 3♣ is a natural call it doesn't make sense to me that is could be a sign-off. But suppose it could be. Then the 3♥ bid has to show 3-card support and game interest even opposite a s/o. In this scenario 4♣ should be a stop signal: We play here. I can't imagine playing like this though. Assuming 3♣ is invitational, 3♥ is accepting and showing 3-card support. In this scenario I can't find any meaning for 4♣ at all. The possible bids should be 3NT, 4♥ and 5♣. Assuming 3♣ is gameforcing, 3♥ must show 3-card support and suggest playing in hearts. In this scenario 4♣ say "We should play in ♣s, not ♥'s" and of course is forcing.