The formula for infinite geometric series is s=a1/1-r. If you would need to find your r, the ratio. The ration in this problem is -1/5. We already know your a1, your first number. Plug everything in.
s= 4/1-(-1/5)
s=4/1+1/5
s=4/(6/5)
s=4*5/6 (multiplying the reciprocal is the same as dividing)
s=20/6
s= 3 1/3