The planned fourth line of Madballs was in the works, as revealed when the third line was showcased at Toy Fair 2009. However, like line three, there has been no word on either line since Feb. 18th 2009. Proof of this series can be found here.
Unfortunately, Basic Fun, one of the companies behind Madballs at the time, had merged with Good Stuff Inc, and Madballs was one of the losses of said merger, bringing an end to Art Asylum's Madballs line.
These are the madballs they were planning on releasing.
The old Madballs were going to be:
The new Madballs were planned to be: