Honestly, I'd probably go with 20mhz too, but is there a difference between a 4mhz and 20mhz crystal?

Let's say you have a standard 50ppm 4mhz crystal.

It's minimum frequency would be 3,999,800   (4,000,000 - .005%)
The max would be 4,000,200 (4,000,000 + .005%)

Now we multiply that times 12 to get the 48mhz and the ...
Minimum = 47,997,600
Maximum = 48,002,400

Check the PPM...
Minimum = (1 - (48,000,000/47,997,600)) = -0.005%
Maximum = (1 - (48,000,000/48,002,400)) = 0.005%

They are still 50ppm difference.

So whether you have a 4mhz or 20mhz crystal, you still end up with the same 50ppm tolerance.
And with 4mhz you also get less emitted radiation, and other PCB problems.

Interesting eh?

DT