I usually make terse responses on the forums to hide my ignorance, but I'll try stepping out a bit here. North has a stiff, 3 aces, and knows they are in a 9 card spade fit. His Axxxx of hearts is really nice opposite a doubleton too imo. Definitely worth more than 2S. Over 2S, from south's point of view, north has some crappy 8-10 count with 3 spades. No way south can move. If north bids 3S, south knows Kx of hearts is golden and can just try and stab a game. Granted, the hands don't have to fit this well, but it must have play. North is closer to just stabbing 4S himself than bidding 2. Edit: oh I suck I thought double was 5S2H, misread the auction. See? I should've just kept it short.