Putting Enough Pressure

This is a deal in the Open League on 2/10/2018.

West Pass	North 1  2	East Pass Dbl	South 1 NT1  ?	Board 21 IMPs N-S Vul	North
1. Forcing					3  Q 10 8 3 2  K J 9  10 8 6 2

Seeing the 9th trump in hand, if partner has about 15 HCP with 2 cards diamond, you would like to be in 4.

Also, with the aid of double, the declarer play will be slightly easier.

An invitational 3 bid would not be wrong but it is not the optimal bid.

By raising to 4 directly, either you may make it or they make a sacrifice which may be phantom.

The drawback is that you may lose to 3=.
However at this vulnerability, the successful rate of the game contract only has to be greater than about 38%.

Putting all these into account, makes 4 a better bid than 3.

The full deal:

West Pass 5	North 1  2  Dbl	East Pass Dbl AP	South 1 NT1 4	Board 21 IMPs N-S Vul	A 9 8 6 4  K J 9 5  A 8  9 5
				K Q 5  7 6 4  10 7 4 3  A 7 3			J 10 7 2  A  Q 6 5 2  K Q J 4
1. Forcing				5 × West Down 2 -300	3  Q 10 8 3 2  K J 9  10 8 6 2