My if-else loop runs only thrice while it should be running more number of times

I have just started learning python 2.7.1, and I have written a code for a Cows and Bulls game, in which you need to guess a four digit number by continuously re-entering 4 digits till you get the right number. But for some reason by code just lasts for 3 loops max. Here is the code:-

number=raw_input("Enter the 4 digit number to be guessed:")
a=map(int,str(number))

def Guess():
    yournumber=raw_input("Enter your number now:")
    b=map(int,str(yournumber))
    i=j=k=0
    while i<=3:
        if a[i]==b[i]:
            j+=1
        elif b[i] in a:
            k+=1
        i+=1
    print str(j),"Bulls and "+str(k),"Cows"    
    return yournumber

Guess()
c=Guess()

if c==number:
    print "BINGO! You have guessed the number!"
else:
    Guess()