I expect a solid heart suit. Since I skipped over spades, it pinpoints the problem. There isn t here. 5C could get a double and they score a club, a heart and a spade. I read partner s bid as a semi-solid suit and no interest in anything more than game.
Partner probably has solid hearts and only one outside card.
This might be enough but probably not. Besides, plus scores at matchpoints are generally good. Slam is possible, but we could have two fast spade losers. The issue is how bad can partner s hand be. If I consider hands he might have for a 4H call, I can construct one that puts 5H in jeopardy, but I can construct a lot more where 6H is cold or on a finesse.