I want to limit the PWM signal for a motor so that it does not spin too fast. I set SERVO4_MAX to 1550 and SERVO4_MIN to 1450. However, when I push the joystick all the way, the servo can still go full throttle.

In this video, you can see the Min and Max values are 1450 and 1550, but the motor seems to ignore those limits.

Just guessing, but did you calibrate your ESC with those settings of Min/Max? I think if if you did the ESC would have 100% range between those points. Maybe try setting min/Max to 1000/2000, calibrate ESC, then reset Min/Max to those and see what happens.