A bid in a new suit when trumps have been agreed that invites partner to bid higher (normally to game) if he is good for his previous bidding but allows room for him to sign off if he is poor. With halfway values, he should judge what to do from his holding in the ‘trial bid’ suit.
You have little in the way of extra values but your heart holding should solidify whatever partner has in the suit. As little as ♠A-K-x-x-x and ♥A-x-x-x-x opposite makes 4♠ almost laydown.