1) Great work by Ray demonstrating convincingly that S3 should not be making this marginal call. The gap in EVs is over a half a point. That's a huge difference. The only argument left is maaaybe this is a good call if S1 is your typical amateur who plays the 2nd round badly, but I highly doubt that will be the case. The gap in EVs is so large here that I think we can safely say this is a pass EVEN in that common case.
2) S4 should lead the JS on trick 2 to try to promote his KS. That line has never been proven to be correct tho.
3) S3 leads the JC on 3rd street to set up an end play. That's expert. Given S1's garbage offsuit lead on 1st street, S3 can already conclude that this partner has no trump, no aces, and thus no help. S3 has to try to get 3 tricks on his own. Well he already got the first trick on 2nd street. Probably the only way he's getting two more is hoping the Right is in the kitty and setting up an end play with K-Left. Leading the Left on 3rd street would be a bad play and a demonstration of poor hand reading. Leading the Left only works well those time's S4 only started with 1 trump, but it's very unlikely that's the case given that we already know S1 had no trump. Again, S3's best hope is to assume the Right is in the kitty and set up an end play and that's what he did.
4) S4 made a very large mistake. S4 has to read the hand better. In this type of spot, after S3 calls, tricks are tied 1-1 and then S3 leads garbage on 3rd street, that will pretty much always mean S3 is trying to set up an end play. With a high trump (AH) behind the maker, S4 CANNOT give S3 what he wants. S4 MUST NOT play right into S3's hands. Holding Ahth, S4 needs to play off in that spot. S4 should also know based on hand reading that his P is gonna pick up this trick almost EVERY TIME. Remember when S1 leads garbage on 1st street he's telling the whole table he has no trump, no aces. So S4 should know that when S3 leads the fresh JC on 3rd street, his P in the S2 is gonna get that trick, and then the maker is caught in a squeeze with S4 holding the AH behind him. BTW even if all S4 had was the AH instead of having AHTH, S4 should still play off for the exact same reasons.