Since it takes 4 clock cycles to complete a single operation, divide the oscillator frequency by 4 to get the number of operations the PIC can perform in a second:

4,000,000/4 = 1,000,000 operations per second.

Take the inverse of the number of operations per second to get the time it takes to perform a single operation:

1/1,000,000 = 0.000001 seconds = 0.001 milliseconds = 1 microsecond.

So yes, four NOPs will delay for 4 microseconds when operating at 4 MHz.

At 20 MHz, each NOP will delay for 200 nanoseconds.

Best Regards,
Paul