(r,theta) in polar coordinates is (rcostheta,rsintheta) in rectangular coordinates.
Hence, (4,pi/12) in rectangular coordinates is
(4cos(pi/12),4sin(pi/12)) or
(4xx0.9659,4xx0.2588) or (3.8636,1.0352)
And, (5,pi/12) in rectangular coordinates is
(5cos(pi/12),5sin(pi/12)) or
(5xx0.9659,5xx0.2588) or (4.8295,1.294)
The distance between the two points is sqrt((4.8295-3.8636)^2+(1.294-1.0352)^2) or sqrt((0.9659)^2+(0.2588)^2) or sqrt(0.93296281+0.06697744) or sqrt(0.99994025)=1
Actually the distance will be exactly 1 as both points have same theta coordinate, bur difference between r coordinate is exactly 1. Minor difference has arisen because of rounding of numbers.