(1) Write a program that will prompt the user for two whole numbers 
    (begin & end), and then print out the *product* of every fourth number
    between the two (inclusive).  Error check user input by ensuring
    that begin is less than or equal to end, and that both numbers are
    a multiple of 4.  On error, print out an informative message and halt
    program execution.
    
    If beg=4, end=12, then the answer is (4)(8)(12) = 384
    If beg=-12, end=-4, then the answer is (-12)(-8)(-4) = -384
    If beg=-4, end=12, then the answer is (-4)(0)(4)(8)(12) = 0
    If beg=12, end=144, then the answer is approx 5.48965 x 10**61