Re: PIC18F: minimum supply voltage for FVR using 4V reference
From Table 27-3 of the data sheet, the tolerance of the reference is only specified for Vdd >= 4.75 V. If i were do such a design I would take Vdd = 4.75 V as a very hard limit if I wanted a guarantee.
There is no guarantee when Vdd is less than this. You don't know how the reference voltage will be behave if Vdd becomes less than this. You seem to assume that there is a gradual degradation of the tolerance. I see nothing that suggests that is a good assumption. For all we know, the reference could stop working entirely at Vdd = 4.7999 V.