B=speed of boat=10mph; C=speed of current.
Effective speed against current is B-C=10-C.
Effective speed with current is B+C=10+C.
Distance=speed*time, so time=distance/speed.
Time against current=4/(10-C)=time with current=6/(10+C).
Cross-multiply: 4(10+C)=6(10-C), 40+4C=60-6C, 10C=20 so C=2mph.