In DragonBall GT, Goku is turned into a child by Shenron. However, later in the series, when he unlocks the power of Super Saiyan 4, he turns into an adult. When he goes back to a regular Saiyan, he's still a kid. Was this ever explained in the manga?
Dragonball GT was never part of the original magna:
Dragon Ball GT (ドラゴンボールGT(ジーティー), Doragon Bōru Jī Tī?, G(rand) T(ouring)) premiered on Fuji TV on February 2, 1996 and ran until November 19, 1997 for 64 episodes. Unlike the first two anime series, it is not based on Akira Toriyama's original Dragon Ball manga, being created by Toei Animation as a sequel to the series or as Toriyama called it, a "side story of the original Dragon Ball". Toriyama only designed the main cast and some machines, and came up with the title
There is no in universe reason given for Goku turning back into an adult as SSJ4, however it hints that:
- He is far more powerful as SSJ4, able to perform attacks and abilities he can't as a kid (Instant Transmission being one of them (Piccolo's Decision).
- It is alluded that the kid body couldn't handle that sort of power.
- Lastly, there is some magic involved in the transformation, from child to ape to man. It is a fully transformed state, which cannot be indefinitly maintained. There are many cases of transformed states looking different to the original. (Buu, Frieza).
The easiest answer is that when he is in his Super Saiyan 4 state he is essentially more powerful than the magic that makes him appear as a kid.
- So as long as he stays in his enhanced state, he stays an adult. As a Super Saiyan 4, Goku is one of the most powerful beings in their universe easily as powerful as any of the wish granting dragons while in his powered-up form.
From the Dragonball Wiki discussing the Goku when in Super Saiyan 4 form:
Super Saiyan 4
This form is achieved by Goku after achieving the Golden Great Ape state and being calmed down by Pan. The overwhelming power of this transformation is apparently great enough to overcome the Black Star Dragon Balls' power making Goku's transformation into a child by Red Shenron.
The spell is only dispersed during the transformation, however. In addition to him being turned back into an adult temporarily, Goku's overall appearance changes yet again. His tail is easily visible, the hair is longer than when in his normal state (not nearly as long as a Super Saiyan 3, though), his eyes become lined (much like Vegeta's did when he was a Majin in the Majin Buu Saga), his eye color is now golden yellow, and red fur covers almost all of his body from head to toe.
His clothes change from his childhood garb, leaving him only dressed below the waist (excluding his armbands). As a Super Saiyan 4, Goku is able to easily surpass all but the most powerful enemies in the final parts of Dragon Ball GT. But at the end of the last episode of Dragon Ball GT, when Goku is an adult again, his tail disappears again and it is unknown if he can ever transform to this state from then on. Other than the fusion with Vegeta (SS4 Gogeta), this is Goku's most powerful transformation.
REF: Further information on Super Saiyan 4 form.
Well,From what I took away from the transformation-Observe the stages of the initial transformation.From Golden ape to ss4.Since he had full control of reducing his size in the transformation ,as he was figuring out how to from into a ss4 he realised he could just reduce himself to a state he remembers himself being(adult) rather than kid.So if you observe the transformation it occurred very slowy forming back into a human form so that was the process of goku forming himself as an adult rather than kid.And after he knew the right feeling to that state ,everytime since then when he transforms he skips the whole ape phase and just jumps to the ss4 form he has now learnt.
If you watch one of the db heroes trailers for ss4,youl see the here(the only place gokus ss4 transformation is shown ever-I mean where he isnt covered by light or rubble) they show him from kid-hez body expands like it bulks up to an adult form then the red hair grows on him.I think thats the most believable way he would do it...they just never demonstrated it in the anime because its not flashy at all