PEF is Photoelectric Effect Factor log recorded by wireline conveyed logging tools.
SNPE is Smoothed Near Detector Photoelectric Effect Factor log, recorded using LWD (Logging While Drilling) tools. Most probably it is a log recorded by Halliburton Sperry Sun drilling services.
Theorectically, both PEF and SNPE should be giving you similar readings.
However, since SNPE being an LWD log as well as using the near detector only, it may have different readings when compared to wireline log PEF. Furthermore, the wireline log can be more affected by borehole conditions and mud cake.
Having said that, the PEF values of 3-4 for shales seem more reasonable than the SNPE values.
Maybe the SNPE log is affected by barite in the mud, making it read higher.
If the PEF or SNPE values are reasonable in the reservoir zones, it should be alright to use both these logs.
I don't think there is a way to convert SNPE into PEF log.
However, you may be able to normalize the SNPE log to a PEF log from a key well.