Generate a 4.25 GHz using 50 MHz crystal/oscillator and PLL
So a PLL uses 2 oscillators. One, the reference, is generally lower frequency, and the other needs to be able to operate at the output frequency. In this case, you'll need a 4.25 GHz VCO as well. Using a PLL IC, the 50 MHz Crystal, and the 4.25 GHz VCO, you'll be able to do what you've described you need. You should download Analog Devices SimPLL application, that will walk you through the calculations. There's no frequncy multiplication taking place in a PLL, only frequency division

National Semiconductor has an Excellent application Note http://www.national.com/en/clock_timing/pll_designbook.html
that covers the details of PLLs. 

The Prescaler details (64/65) for example are called "Dual Modulus" prescalers, and they work in conjunction with another divider to provide the correct divide ratio.