Hi,
You normally tell proteus what oscillator speed you want by right clicking on the pic chip, in the properties box enter your oscillator speed.
If you connect a crystal or R/C to the osc pins in ISIS it will simply ignore them when running a simulation, it relies on the Properties box value.
Of all the oscillators you could use, a R/C is the least stable.
Being as that chip has a very good internal 4mhz oscillator why not use that ?
For a real pic chip you must set it to the Internal osc in the config code.
For Proteus just specify 4mhz in the properties box.