Both are of little consequence.

The OSCAL value trims the INTERNAL Oscillator. This is only important for timing critical functions. Set the CONFIG fuse settings to give you CLKOUT on GP4 and play with the settings until you get as close to 4MHz as possible using a Frequency Counter.

Alternatively, DEBUG out a string like "abcdefghijk" followed by a CR/LF, in a continuous loop starting at 300 baud, and adjust the OSCAL values until you get something sensible on Hyperterminal. Once you've got it working, increase the baudrate to 2400 or 9600 and repeat for a finer trim setting.

If you don't plan to do timing critical functions it's a complete waste of effort. Get a life, get a beer, get anything, damn PIC costs less than a Dollar USA and how many hours did you plan to spend on this?

The BANDGAP just trims the BOR and POR values. Set the PCON register to enable BOD, have a simple LED program in a loop. Use a variable power supply along with a good DVM to drop the supply voltage until the PIC trips-out. Trim the bandgap until the PIC trips within the value range in the published specifications. A lot of hassle to trim a few millivolts here or there.