I'll try to explain more. Anyoone can feel free to add.

Your PIc has 2 types of memory. Program and data memory. Your program is compiled and loaded int o program memory. Your data is stored in data memory which is volatile memory meaning when you power off your PIC, it is erased.

Your memory is divided into addresses. The last address in your program memory has a value put in by Microchip. This value calibrates your PIC as close as possible to the clock specified in the datasheet. If you erase your chip, you lose your calibration value. So the first thing you should do when you put a PIC in your programmer is read it and write this value down on paper in case you overwrite it by accident. This value is unique to each PIC.

What i'm not so sure of is, will this value make your PIC go to 4.00000000 MHz or will it be off a little. Maybe someone else can answer that????? Is this value good enough for serial communication?