Since, in GT, Goku wished to be 10 years old again, he lost a lot of power, but not knowledge. At, 10 years old, Goku had enough power to blow up the moon which is 1.25% of the Earth. Since, by most fans, SSJ4 is said to be 4,000x base form. He could blow up Earth 50 times. If a power level of 180 can blow up the moon, Master Roshi, and the moon is 1/80 the size of the Earth, that rounds to 15,000 power level. 15,000 x 50 = 750,000 power level. This explains most of GT's inconsistencies, since base form Goku is known for being in the 1million range in DBZ. Hope that helped.