help with macro to save repeating data entry

[Tom]

hello

have the following 4 fields in excel

f1 f2 f3 f4

A 2 9
B 3 8
A 2 9
n 4 6
A 2 9

what i need is a macro that when a user enters a value in the first
record for eg, no 44 in f4, I need the macro to search thru all
remaining records which have the same value as that in the data entry,
(in this eg, A 2 9) and populate the f4 field with the same value
being 44

f1 f2 f3 f4

A 2 9 44
B 3 8
A 2 9 44
n 4 6
A 2 9 44

hope u can help
tom