I have used C4 and C4P in my 59AB filter housing. I'm not sure what the difference is between the two, but one must be better than the other, because it has more letters.
Actually, I believe the difference is in the filter media. Some specs below.
The description of the C4 is as follows:
Part Number C4
Product Type By-Pass Lube Cartridge
Product Height 4.88"
Product I.D. .53"
Product O.D. 4.2"
And the C4P is listed as follows:
Part Number C4P
Product Type By-Pass Lube Cartridge
Gasket I.D. 4.34"
Gasket O.D. 4.73"
Gasket Thickness .08"
Gasket Usage Gasket
Product Height 4.88"
Product I.D. .55"
Product O.D. 4.33"