In this case, we'll have to carry out several steps to find the solution. 
Step 01:
Data:
 (x - 4)² + y² = 16
Step 02:
polar form: 
x = r cos (θ)
y = r sin (θ) 
(r cos (θ) - 4 )² + (r sin (θ))² = 16 
(r cos θ - 4)² + r² sin² θ = 16
r (r - 8 cos (θ)) = 0 
r = 8 cos θ
The answer is:
r = 8 cos θ