Phrase 3 is perfect.
Phrase 1: it's debatable whether or not this phrase is grammatically valid. I will say it's technically valid, but has poor grammar nonetheless.
Phrase 2 is the only one that's undeniably wrong grammar, but it can be fixed by changing "as" into "as to", or into "of" as in phrase 3.
Phrase 4 can be grammatically be fine, but has a different meaning - it bakes the question "which question about it?". For this reason, if what you intend to say is equal to phrase 3, then phrase 4 is saying the wrong thing, so it becomes even worse than phrase 2 which has mistaken grammar but is still easily & accurately understood.
ps. I wrote this without reading what anyone else said. I'm pleased to see Jonathan put them in the same order, and has pretty much the same explanation as me for phrase 4 being last.