NORTH J2 AKT963 AT73 9 985 85 KQ852 QT7 7 J42 J96 K86532   AKQT643 Q7 4 AJ4

Dealer: West    Vul: Both   MP Scoring West Pass Pass Pass Pass Pass North 1 2 3N 5 Pass East Pass Pass Pass Pass Pass South 1 3 4 6

South's 3 is game forcing. North had room for a 3 bid, but a lead coming into those diamond, an anticipation with minimum opening, overuled.
When South bids 4 opposite a possible North spade void, North's singleton club revalues to at least 2 and in total the 15 suggests at the very least a try at slam given a South without slam interest and such dependable spades would have leapt to a spade game after 2.

After such showing, the already conservative 4 spade bid needed nothing further ask.