=MATCH(AI4,QUARTILE(AI$3:AI$67,{4,3,2,1}),-1)

I tried using the following equation but it returns an occasional blank:

=MATCH(AI4,QUARTILE(AI$3:AI$67,{1,2,3,4}),1)

Truthfully, I'm not entirely sure how the above equation works, but I tried to do the inverse.