PROGRAM prod4
   IMPLICIT NONE
   INTEGER :: k, beg, end
   DOUBLE PRECISION :: prod
!
!  Get begin & end
!
   PRINT *, 'Enter begin and end (both mult of 4) separated by space:'
   READ *, beg, end
!
!  Do error checking
!
   IF ((beg/4)*4 .NE. beg) THEN
      PRINT *, 'Begin must be a multiple of 4, but you entered ', beg
      STOP
   END IF
   IF ((end/4)*4 .NE. end) THEN
      PRINT *, 'End must be a multiple of 4, but you entered ', end
      STOP
   END IF
   IF (end .lt. beg) THEN
      PRINT *, 'END must be greater than or equal to BEGIN!' 
      STOP
   END IF
!
!  Figure product using counter loop
!
   prod = 1.0
   DO k = beg, end, 4
      prod = prod * k
   END DO

   WRITE(*,1000) beg, end, prod

1000  FORMAT('Product of every number in [', i4, ',', i4, ':4] is ', ES31.23E3)
END PROGRAM prod4