Need Help with SQL

Hi,
I need some help with sql.
My table has 4 fields: A, B, C, D
I need my output to be grouped by A, B, C and a value of D which occurs
maximum number of times (if more than one value occurs same number of
times, any of those values is fine).
So if my records are:
A1, B1, C1, D1
A1, B1, C1, D2
A1, B1, C1, D2
A1, B1, C1, D3
A1, B1, C1, D4
A1, B1, C1, D4
A2, B1, C2, D1
My output should be:
A1, B1, C1, D2 or it can be A1, B1, C1, D4
A2, B1, C2, D1

This would be a one time process on about a million records so performance
is not an issue and it need not be a single sql.

TIA
Vinay