Partner needs three top tricks from you to make his bid, and four to make game. It is likely the opponents will collect four tricks—possibly as many as six if the diamond ace is offside—before your high cards can come into play. Another reason for passing is that the opponents may be goaded into rash action by the preempt. Still, four spades might meet with good luck. Three no trump gets a sop but is unlikely to succeed.
Since partner should hold seven hearts, it is almost even money (48 to 52) that you will not score a defensive trick with your ace. Therefore, you should aim to keep the opponents from bidding their game or, quite possibly, a slam. How best can you do this? The imaginative bid of four no trump (Blackwood) may silence East-West if their strength is divided. At the same time, it ensures that you will get no higher than five hearts, which should be a profitable sacrifice. Partner might construe a five-heart bid as a real slam try, asking the quality of his trump suit. Four hearts could result in a bargain if each opponent believes you hold the high cards he is lacking. The only thing you must not do is pass.
You want to play in three no trump unless partner's hand is so good that he can move toward slam or so poor that he rescues with some distributional values. Doubling for takeout creates problems if partner responds in diamonds. Four clubs runs three risks: a penalty double by West, a missed game—perhaps in hearts—or playing for game in the wrong suit. A pass gets one point, for on rare occasions it may avert disaster.