When will Goku die forever

DBGT: Will Goku die of old age if he turns SSJ4 in the future?

No. The wish was to get Goku back to one Child too do. If he again grows up , the request will no longer come into effect. The youth he wins would be just a side effect of the desire. The wording of a wish is very important, as can be seen when Shenron granted a wish to resuscitate everyone on Namek killed by Frieza and his henchmen. That wish didn't revive the people Vegeta killed because Vegeta broke off from Frieza.

Assuming the wish is still considered in effect, in the transformation it would still not become an old man. The reason he grows up is because of that negative effects of wanting to negate his power. As soon as he's grown up again affects the desire actually positive, because it is in its prime. During the Saiyain saga, Shenron told everyone he couldn't use his magic to kill Vegeta and Nappa because they were too strong. At the same time, we have no doubt that Shenron could immortalize the Saiyains if their plans to get the Dragon Balls were successful. So it's not just about Shenron's magic not affecting people stronger than him. It's also about whether the person wants to be affected. This can be seen explicitly when Porunga cannot bring Goku back to Earth after Namek explodes because Goku refuses.

If you remember, King Piccolo wanted to restore his youth, but he was also stronger than Shenron, as evidenced by his ability to destroy the dragon. The wish still worked.

In case anyone is confused, all of the examples I've used are from the Shenron and Porunga original, but they should apply to Ultimate Shenron anyway. The only difference is that Ultimate Shenron is stronger, so it requires more power (Super Saiyain 4 level) to overwhelm its magic.

Torisuda

The mechanics of Shenron's magic are one of the most tangled, inconsistent things in the entire tangled, inconsistent DB universe, but you've brought in some good examples to prove your point. +1.