The 4th Robo has been revealed through info:
A battle ship/delta-shaped jet vehicle, who greatly resembles TimeJet Gamma from Timeranger. It's coloration is white, with red lines, giving it similarity to J-Der/J-Ark from GaoGaiGar. Ultimate transforms to its robot form, and has the Super Goseiger's 5 Headders affixed to its chest in a Star Formation, over a hexagonal-face.
GoseiUltimate can combine with GoseiGreat to form Ultimate GoseiGreat. Other gattai abilities are hinted at, but the full catalogue has apparently not been released, as Ultimate would only cover the month of October. As per usual oder, we can expect Bandai to release toys in September, October, and November.