First: you will need to place the complex number -2·sqrt(3) - 2i into r·cis(theta) form.
For the complex number a + bi r = sqrt( a2 + b2 ) and theta = tan-1( b/a)
For your problem: a = -2·sqrt(3) and b = -2:
r = sqrt( ( -2·sqrt(3) )2 + (-2)2 ) = sqrt( 12 + 4 ) = sqrt( 16 ) = 4
theta = tan-1( -2 / (-2·sqrt(3) ) = tan-1( sqrt(3)/3 ) = 7pi/6
r·cis(theta) = 4·cis( 7pi/6 )
General rule: [ r·cis(theta) ]n = rn·cis( n·theta )
Now, to find the 4th power, take the r-value to the 4th power, and multiply the angle by 4.
[ 4·cis( 7pi/6 ) ]4 = 44·cis( 4·7pi/6 ) = 256·cis( 14pi/3 )
(You'll have to reduce the 14pi/3 into an angle smaller than 2pi.)