Power increases linearly with switching speed, so 20 MHz will
draw ~5 times as much current as 4 MHz, but will also do the
job 5 times faster.  I'd bet that the overall energy consumption
wouldn't differ too greatly, with the 4 MHz having a tiny lead
due to lead and source impedance losses of the higher current
(the whole I^2*R thing).

The important thing to consider is peripheral operations.  At
20 MHz, your peripherals (external ADCs or memories and such)
will be on for a much shorter time, because you'll be able to
read/write to/from them much faster (assuming you're using
SPI; I2C probably maxes out well before 5 or even 1 MHz).

This maybe a gross over simplification.  Other factors to consider
are data transfer out of the device (USART), EMI and sampling
rates.  Finally, consider using a PIC18F series that can have the
clock switched between a relatively slower internal RC and a
faster external oscillator for USART transmission.

Mike H.

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