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.
that's for sure

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.

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.
Yep it's my assumption too or worst... ther'e the same only the stamp is different... i hope everything will work for awhile.

BTW keep us post on it.