Sometimes it easier to see the correct answer when we use another word in replacement for the one we are having difficulty to understand.
If only we ______________ a three instead of four. We'd be rich!
decide; had decided.
If only we decide a three instead of four. We'd be rich!
This cannot be correct: in this context we will be rich if we decide 3 not 4.
If only we had decided a three instead of four. We'd be rich!
Had is referring to something that was possessed in the past...but
not now.
in this context we would have been be rich if we decide 3 not 4.
choose:
chose | chosen Ref C.E.D.
to decide what you want from two or more things or possibilities: