3♠ is surely a heart raise even with no agreements after opps bid to 4♠ and we have 3 small spades. My first thought was 5♥ (minimum, but no interest in defending 4♠), but then I noticed we have 7 hearts not 6 (AK5 not AK4), plus partner has made a strong bid. Perhaps partner has a very strong hand but is worried about diamonds - either the King which will be led through at trick 1, or just 2 small diamonds (with a bunch of card outside diamonds), he won't move over 5♥. We have a diamond control. I like 4NT RKCB now. If partner has a spade void and no ♦A we need good methods, but with most hands we should be able to get to the right spot.