python exercise with multiple

I'm newbie. I'm trying to find out a solution to an exercise but i'm not so skilled and I made several attempts with no results.

I have to write sort of a game. To start, the computer has to ask you to decide a number X. If the number is a multiple of 4 the user starts. Otherwise the computer will. Taking turn, computer and user subctract a number from 1 to 3 to "X" and get a value Z. The one who wins the game subtracts the latest balls and lets with no balls to pick the other contestant (in this case the user). I have to write everything in a fashion that the computer will get the wins, always. Below, the code I have so far written.
Any hint?

x = int(input("how many marbles?"))
if x%4==0:
    print("you start")
    y = int(input("subtract 1 or 2 or 3 marbles: "))
    z=x-y
    print("there are ",z," marbles left")
else:
    print("computer starts")
    if x==x+3:
        m=x-3
        print("there are ",m," marbles left")
    if x==x+2:
        m=x-2
        print("there are ",m," marbles left")
    if x==x+1:
        m=x-1
        print("there are ",m," marbles left")