The thing I like about computers is that thery're pretty idiot proof if you think about it - especially the mnemonics...

FOSC = Frequency OSCillator

Namely it's the frequency of your xtal/resonator - ie the Fequency of the Oscillator.

Whereas Fosc/4 is the instruction speed.

btw... I calculate 10 bits to be precisely 3906.25 at 4.000000MHz assuming you have a xtal that's spot on *smiles*...