The difference between 4w AM and 8w AM...is less than a needle's width on the receiving set's S-meter, and often places the active devices dangerously close to the point of self-destruction in a situation with a high VSWR.

There is no reason whatsoever to run more than the legal power level in the Class D CB service, and no more than 85-90% modulation.

If you can't cut the mustard with those parameters it's time to look into more antenna.