-- Google Sheets: Using filter()
--   in custom formula in data validation

=query(filter('sheet1'!$A$2:$A, 'sheet1'$A$2:$A < 4),
       "select Col1 where Col1 = '" & 'sheet2'!A2 & "'")
       = 'sheet2'!A2

-- Note: The 'sheet2'!A2 must be
--   the first (top-left) cell of the validated range