4♥ is systemically to play, but it seems likely from your hand and from previous experience that partner has forgotten this agreement. 1♠ (dbl) 4m would have been a fit bid; the only splinters that you play in competition are in an opponent's suit.
You are using screens. Your screenmate is to your right.
Which of these would be permitted:
(1) Tell RHO that 4♥ is supposed to be natural, but partner almost certainly has either a fit bid or a splinter, then bid 4♠.
As I understand it, the EBU penalises fielded misbids by inferring a concealed partnership understanding. If you explain both the actual agreement and what you think partner has got, does that get you off the hook?
It is permitted in the EBU to play partner's 4♥ bid as any game-forcing meaning, so it's legal to play it as a three-way bid - natural, a fit bid or a splinter. Hence the question is only one of disclosure. On the other side of the screen, of course, the explanation will have been different.
(2) Bid 4♠ as a cue-bid in support of hearts.
(3) Raise 4♥ to 5♥ preemptively, then pass partner's conversion to 5♠.

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