I routinely use the PIC 18LF4620 at 3.7 down to 2.5 volts. I run at 4 MHz and the config data I load into the Melabs USB serial programmer is:-
Code:
'************* Config Register setup for USB programmer ****************
' OSC = HS,  FSCM = En, IESO = En, PwrupTmr = En, BrownOut = Dis
' BOR Volts = 2, WDT = Off, WDTPS = 1:512, CCP2 mux = RB3, 
' PortB rst = Digital, LPT1Osc = Low, MCLR = Reset, LVP = Dis
' Enhanced CPU = Dis, All memory = Not Protected
' Note re power consumption.    PBP assumes the WDTPS is set to 512.
' SLEEP draws 2.5 uA in this system.  Each time sleep times out there
' is a current pulse for 2.5 mSecs at 600 uA.  Setting WDTPS = 1024 extends
' the SLEEP internal period by 2 and so the average power drops BUT
' PBP will get the sleep times wrong by the factor of chosen WDTPS:512
'************************************************************
Do NOT enable LVP which is the short life option - you will go mad and die young trying to find intermittent programming problems.

Even when the WDT is disabled, the WDT post scaler is active and to get the sleep times right you must set the WDTPS to 512.

Enabling the WDT adds 2 uA to the sleep current.
Enabling the BOD also adds about 2 uA to the sleep current.

HTH
BrianT