You can only control two servos independently with the Pi's hardware PWM.
GPIO 12/18 share a channel.
GPIO 13/19 share a channel.
The same settings apply to GPIO which share a channel.
E.g. If servo1 is on GPIO12 and servo2 on GPIO18 they will get the same signal (same frequency, same pulsewidth).
pigpio supports the hardware PWM features of the Pi. So you ...
A pulse width of 1500 µs moves the horn to the centre. You can call that any angle you want, say 0.
For each 10 µs decrease in pulse length the servo moves one degree clockwise.
For each 10 µs increase in pulse length the servo moves one degree counter clockwise.
These figures are approximate and vary by servo model.
The servo will have angle limits. If ...