What is difference between stock CDI and racing CDI in a 4 stroke motorcycle?

A CDI ignition system uses capacitors to charge up the energy for the spark plug discharge. Generally speaking, capacitors do this more quickly than electrical coils, allowing the ignition system to recharge more quickly for another cycle, meaning you can achieve higher RPMs.

That being said, not all capacitors are the same, and some charge more quickly than others. The ones that charge more quickly will allow even higher RPMs, if your engine can handle it, whereas capacitor that charge more slowly would need more time between cycles, meaning lower max RPMs.

The stock CDI would have slower charging capacitors with a lower max RPM, while "racing" CDI would have faster charging capacitors and a higher max RPM.

You won't notice a difference at all unless you push the engine to the rev limit, assuming your engine can reach the higher rev limit allowed by a "racing" CDI.