The fourth season of the animated television series Johnny Test premiered on November 10, 2009, containing 26 full episodes.

"Johnny's New Baby Sisters": Susan and Mary purposely revert themselves back into babies while the parents are out, and Johnny has to keep them safe from the irresponsible babysitter Gil; this interferes with a party he and Dukey had been planning and Mr. and Mrs. Test are mad.

So Susan and Mary switch Johnny and Hugh's brains after the two make a bet that involves taking on each other's lives.

The sisters and Johnny try to find outdated technology to power the tea house.

More methane from the train is eventually obtained to power the machine and get back to the present.

They spit meatballs at Hugh, brake Lilas car, and trash the lab.

"Johnny's Amazing Race": Johnny and Dukey challenge his sisters, their friends, and their enemies to a survival challenge in which the winning team gets hundreds of dollars in gift certificates.

"Guess Who's Coming to Johnny's for Dinner": Mr. Black moves in with the Tests after he embarrasses Mr. White at the dance contest auditions.

"Sleepover at Johnny's": In a parody of The Cat in the Hat,[16] Johnny is dragged into Susan and Mary's plan to get Gil to be in a sleepover, but Bling-Bling Boy and Dark Vegan's daughter Jillian become involved as well, with usual chaos ensuing.

"Johnny's Royal Flush": When Susan and Mary turn Johnny into a fish as part of one of their experiments, Hugh flushes him down the toilet by mistake where he ends up in the sewer, and he has to swim for his life when he runs into an alligator.