Is the number 4 only likely to occur once in your detail sheet? If so,
you can use the VLOOKUP( ) function in the summary sheet to look for a
match in the detail sheet and return a corresponding value from the
same row. I'm not sure, though, what your example summary sheet is
meant to represent, or how the letters are related to the detail sheet.
If you can come back with some further details then i will be able to
elaborate on my suggestion.

Pete