* 4KidsTV is no longer in operation: The company ceased operations in 2012, so they wouldn't be able to produce any new dubs.
* Licensing Rights: The rights to Yu-Gi-Oh! GX are now held by different companies (likely Konami). Even if 4KidsTV was still around, they would need to obtain new licensing agreements.
* Dubbing Costs: Dubbing an entire anime series is expensive, and given that the show has already been dubbed by another company (in this case, 4Kids itself), there's less financial incentive for them to do it again.
What happened to the English dub of Yu-Gi-Oh! GX?
4KidsTV only dubbed the first three seasons of Yu-Gi-Oh! GX. The English dub of season 4 was handled by a different company, Toonzai (who also handled the dub of the 5D's series).
Where can I find the English dub of Yu-Gi-Oh! GX season 4?
You can find the Toonzai dub of Yu-Gi-Oh! GX season 4 on various streaming services, including Crunchyroll and Hulu.
Is there a possibility of a re-dub of Yu-Gi-Oh! GX in the future?
While it's not impossible, it's highly unlikely. However, if the demand is high enough and the licensing rights can be secured, it could be a possibility in the future.