#### Answer

$(x-2)^2+(y+1)^2=16$

#### Work Step by Step

RECALL:
The standard equation of a circle whose center is at $(h, k)$ and has a radius of $r$ units is:
$(x-h)^2 + (y-k)^2 = r^2$
The given circle has its center at $(2, -1)$ and a radius of 4 units.
Thus, the standard form of the circle's equation is:
$(x-2)^2+(y-(-1))^2=4^2
\\(x-2)^2+(y+1)^2=16$