I play that 3C shows extras with 5-4 or better. Responder bid the fourth suit which I take initially as a try for 3NT. She might have a 2353 hand with no heart stopper for example. Opener's 3NT is typically either 5224 or 5314. Responder's 4D tells us that she has either lots of shape that doesn't want to play 3NT or a slam try hand. While its true that 3H can be bid as a 3NT try without holding 4 hearts, in this auction responder has hearts for sure as she failed to bid 3D. Typically, she would have 6-5 in the reds. Using the game before slam principle, 4H shows a 5314 hand offering 4H to play.