DBGT: Does Goku die of old age when he turns SSJ4 in the future?
No. The wish was to make Goku a child again. When he grows back into an adult, the wish will not be in effect anymore. The youth he gains would just be a side effect of the wish. The wording of a wish is very important, as can be seen when Shenron granted the wish to revive everyone on Namek killed by Frieza and his henchmen. That wish didn't revive the people Vegeta killed because Vegeta broke off from Frieza.

Supposing the wish is still considered to be in effect, he still wouldn't become an old man when transforming. The reason he becomes an adult is to negate the negative effect of the wish on his power. Once he becomes an adult again, the wish actually has a positive effect because it is keeping him in his 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 don't doubt that Shenron would be able to make the Saiyains immortal if their plans to get the Dragon Balls succeeded. So, it isn't just a case of Shenron's magic not affecting people who are stronger than him. It is also a matter of whether the person wants to be affected. This can be explicitly seen when Porunga is unable to bring Goku back to earth after Namek explodes because Goku refuses.

If you recall, King Piccolo wished to restore his youth, but he was also stronger than Shenron, as evidenced by him being able to destroy the dragon. The wish still worked.

Also, just in case anyone is confused, all the examples I used were from the original Shenron and Porunga, but they should still apply to Ultimate Shenron. The only difference is that Ultimate Shenron is stronger, so it requires more strength (super saiyain 4 level) to overpower his magic.