What's in a signal?

Playing a top-bracket knockout at the Washington NABC's, red vs white, you look down at

65

J742

AJ86

AQ5

Playing a light, aggressive opening style, partner deals and passes and your RHO open 1S. First choice, pass or X?

Since you are outranked by the boss suit, you decide to pass and the bidding continues with 3H, alerted, on your left and 4S from opener. 3H is explained as being an artificial limit raise in spades and you decide to lead the 2 of hearts. Your agreement is 2nd/4th, Polish-style, so this promises an honour (4th) or is the lowest from xx. Dummy hits and this is the view:

_______AQ84

_______87

_______K97

_______T754

65

J742

AJ86

AQ5

The 2 draws the Q from partner and declarer wins with the A. Without further ado he draws trumps in two rounds ending in his hand. Partner follows low once and then discards the 5 of diamonds, playing upside-down count and attitude (UDCA). Declarer now leads the 4 of diamonds from his hand and we do what??

Initial analysis shows 8 hcp in the majors from declarer and a reflection that any late heart losers can be ruffed in dummy. 6 spade-tricks, HA, maybe a H ruff in dummy and the K of diamonds brings up the total to 9. If declarer has the Q of diamonds we can duck now without risk (not going anywhere) and get some more info. If partner has the Q, declarer might have a stiff and we need to grab the ace immediately.

What can we deduce from that diamond discard?