Greenman vs. Gaira (グリーンマン対ガイラ, Gurīnman tai Gaira?) is the fourth episode of Go! Greenman.
The episode begins with six children playing hide-and-go-seek at the playground. Unbeknownst to them, Tonchiki and a Minion of Maoh are watching them in a bush, and they decide to sneak up on the child who is counting. They both run into the child, revealing their presence. They try to chase the other five kids, but they begin to throw rocks at them. Disappointed with Tonchiki, Maoh teleports him back to the Underworld and tells him to mutate the Minion into a monster. So, Cretin does so, and the Minion is transformed into Gaira. Seeing the monster, the children run away and call for Greenman with the Green Calls. Greenman quickly appears to stop Gaira, and they begin to fight. After being thrown around multiple times, Maoh tells Cretin to make the monster grow. Cretin does as he is told, and Gaira grows to giant size. Greenman also grows to continue the fight. Gaira proved to be quite strong, as he was able to pick up Greenman and throw him. The monster was able to drastically weaken Greenman, and nearly threw him off a cliff. Luckily, he was able to push Gaira back away from the edge of the cliff, until there was enough space to continue fighting. The children saw that Greenman was failing, so they used to Green Calls to strengthen him once more. Greenman summoned the Greenman Stick and finished Gaira off with its explosive tip.