I have a project numbers which is split up over two columns. The first 4
digits (Col B) is the number for the parent project and the next 3 digits (C)
is the subproject number for that project.

We anticipate this spreadsheet being large so I would like to create a
formula at the top of the spreadsheet that shows the next available number so
that the user does not have to search. Finding the next parent project is
easy (=MAX(B#:B#) +1) but what I need to do is have the user enter a parent
project number (cell A4) and the formula showsl him the next available
subproject number for that parent project. Any ideas.

Eg. the user enters 0054 as a parent project (A4) and the formula would
display 005 which would be the last subproject for project 0054.