Dealer with

after picking up the 10D. Spades led, S4 trumps with 10D. What to lead, trick 2?
lead 9H: EV = -0.56
lead KD: EV = -0.53
61,482 hands make it to dealer for a decision; 19,466 times S4 wins 1st trick w/ 10D
lead AC: EV = -0.41
lead 9H: EV = -0.47
61,128 hands make it to dealer for a decision; 19,380 times S4 wins 1st trick w/ 10D
lead AC: EV = -0.41
lead KD: EV = -0.54
61,485 hands make it to dealer for a decision; 19,457 times S4 wins 1st trick w/ 10D
So it seems leading AC is the best option.
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Dealer with

, turn card is

.
S4 calls picks up 10D, discards 9S. S4 wins the 1st trick w/ 9D, what to lead, 2nd trick?
lead AC: EV = +0.10
lead 10D: EV = +0.20
73,260 hands make it S4 for a decision; 22,705 times S4 wins 1st trick w/ 9D
Obviously a better hand (3 trump), so S4 calls trump more often and has a better EV.
lead KD: EV = +0.16
lead 9H: EV = +0.09
73,244 hands make it to S4 for a decision; 22,857 times S4 wins 1st trick w/ 9D
lead 10D: EV = +0.21
lead KD: EV = +0.18
72,947 hands make it to S4 for a decision; 22,629 times S4 wins 1st trick w/ 9D
So it seems leading 10D is the best option.