Watch The Big Bang Theory Season 4: Episode 16: The Cohabitation Formulation Online where we shall see that Wolowitz and Bernadette are in a great spot in their relationship but he never stays over because he always has to run home to help his mom out in the morning.
Bernadette wants them to move in together but Wolowitz can't see the two of them living with his mother working out so well. Finally, Bernadette has enough and gives him an ultimatum - her or his mother. Howard chooses Bernadette and shows up at her door all ready to move in.
Unfortunately, what Bernadette quickly discovers is that Howard didn't quite leave his mother behind - he just replaced her with Bernadette! He expects her to make him dinner, take him to the dentist and do everything his mother does. When she calls him on it, Howard thinks she's being ridiculous and hightails it back to his mother's.
Meanwhile, Priya, Raj's sister is back in town and reunites with Leonard, even though Raj forbids it. Penny meets Priya, and Amy Farrah-Fowler, as a witness to all of this, offers her shoulder for Penny to cry on as she understands how hard it must be to know Penny's been replaced by a much better and more suitable mate for Leonard.
At first, Penny feigns indifference but eventually she breaks down, much to Amy's delight as she can use Penny and continue her study of how intense crying and emotions are triggered in the brain.
