Answer:
2
Step-by-step explanation:
Given the circle: 
To determine the length of the radius, we complete the square.
Step 1: Subtract 4 from both sides

Step 2: Divide the coefficient of x and y by 2, square it and add it to both sides

Step 3: Factorize the Left Hand side and simplify the Right Hand side.

Comparing with the standard form of the equation of a circle

r=2
Therefore, the radius of the circle is 2.