Don't worry about the pitch, 48dp is absolutely fine and changing wouldn't benefit you much. It depends a lot on your individual track, motor etc as to exactly what pinion you run. On a medium length track, probably something between 24-28t would be a good starting point.
Ask other drivers at your club what gear ratio they use. You can work out your tc4 ratio by doing: (teeth on spur/teeth on pinion)*2.5. The 2.5 bit is the 'internal ratio' on the tc4 that is created by the different sized gears in the transmission cases. A smaller pinion will give you a higher number gear ratio, and will increase the cars acceleration, but lower the top speed.