Ok I think I figured this out.
I added a filed suffix and now after I select the project combobox i do
Private Sub p_AfterUpdate()
If IsNull(Me![s]) Then
Me![suffix] = Format(Nz(DMax("[suffix]", "[Table4]", "[project]='" & p & "'"), 0) + 1, "00000")
End If
Me![s] = Format([suffix], "00000")
End...