what is the maximum range of citizens band radio computation
At sunspot maxima, propagation to the antipodes at 20+MHz is certainly achievable with 4W, and signals at the receiving end can be surprisingly strong (50uV into 50 ohms is not unusual). Radio hams do it all the time!

With 100W, earth-moon-earth (EME) is achievable at higher frequencies, using the moon as a passive reflector, although higher powers (~1kW) are normally used.