There are several reasons for asking this:

50% of the Boards have already been manufactured and and have a 4MHz chip on board.
Attaching a 8 MHz Resonator (Pads are there) is a lot easier than replacing the PIC.

I have got a "special offer" for the 4 MHz SMD version,
it is really half the price of the 20MHz version.

The higher clock speed is a must for capturing synchronus serial data on this project.
(reducing comms speed is not an option)

The board I'm using on this project was initially designed for a different project where 4MHz was sufficient.

In the meantime I have done some testing
and it turned out that the 4MHz chips run reliable up to 12 MHz,
in a temp range of 0 - 50 deg C.
(I haven't tested any higher clock speed or temperatures)

As I'm planning to run the PIC at 8MHz
in a temp range of 25 (+-10) deg C "only" I think its worth giving it a try.

I totally agree,
it would be better to run all components within the specs, but this time I do not really have a choice as cost is THE key issue.

P.S.

I dont know how PICS are tested at MicroChip,
but lets assume there is no separate production for 4MHz Chips,
it is just a question of did the chip pass the 20MHz test.

case yes stamp 20MHz onto the chip

case no stamp 4MHz onto the chip

If this assumption is correct
I will be running a chip that was designed for 20Mhz but did not pass the test
at "only" 40% of the speed it was initially designed for.

regards

Ralph