Grey's Anatomy - Season 4

Callie
Callie has thinks she might be pregnant but George doesn't care 'cause he is in love with Izzie.
She struggles with being the chief resident, and Bailey is doing all her work.
She leaves George after figuring out that he was cheating on her.
She argues with Izzie, and almost/sort of/not really fights her in the cafeteria.

(Maile Flanagan guests in this episode!)


Eventually the chief catches onto the fact that Bailey is covering Callie's ass and demotes Callie and gives the chief resident job to Bailey.
George's mum visits with baby clothes for Callie (she doesn't know they broke up) Izzie soon clears that up, albeit accidentally.
I have no idea where the "Callie might be pregnant storyline goes".



Erica
Erica is invited to perform a heart transplant at the hospital in episode 5. She doesn't get along with Yang, and chooses Izzie to scrub in on the surgery instead. The chief also offers her a job as the new head of cardiothoracic surgery and Erica accepts.
She crashes the chief's "gentleman's evening" with Derek and Mark, they all play board games together.
Mark flirts with her. A lot. She shoots him down every time.
She eventually lets Yang in on a surgery after Yang takes advice from Callie to stop being such a kiss-ass.
Erica reaches out to Callie and Mark in episode 10 and asks them if they want to go for a drink after work. The 3 of them have some pretty good banter.
Tucker, Bailey's kid gets trapped under a bookshelf and Erica has to perform life saving surgery on him.
The chief wants her to do a very risky operation on his mentor, and although she initially refuses, she eventually agrees and the operation is a success.



Gay-ness
Begins when Erica invites Mark and Callie out for drinks after work in episode 10.
Yang gets all jealous about Callie hanging out with Erica all the time and the fact that they are becoming fast friends.
Some of the Best. Dialogue. Ever. happens when Addison returns and questions Callie about her relationship with Erica. Callie insists she is a huge, huge fan of penis and tells Addison she's been living in LA too long.


(Side note: Addison looks pretty damn good whilst she's eating whatever it is she's eating in that clip)

Erica lets Yang in on a surgery after Callie agrees to buy her a drink or two later that night.
Callie freaks out when Erica is nice to her and removes a hair stuck in her lip gloss or something. She runs off and sleeps with Mark.
She becomes all distant and Erica calls her out for avoiding her. Callie admits she was a little weirded out by the fact Addison mistook them for a couple. The two share some very awkward/nervous laughter.
They mess with Mark's head and joke about threesomes and being a couple. Erica kisses Callie in the elevator to show Mark that he couldn't handle a threesome with them. Although going by the look on Callie's face afterward, she couldn't handle the kiss


Season 4

Her and Mark continue to sleep together, with Mark talking dirty about Erica.
Callie begins to get flustered around Erica and Bailey tells her "You're acting weird".
Mark messes with Callie's head but eventually calls her out on the fact she's obviously into Erica and tells her to "finish what you started".
Callie kisses Erica. Mark watches rather creepily from the sidelines for a little bit before walking off. Although it's clear he just wants Callie to be happy.



Couple of videos worth checking out are these slightly extended versions of a few of their scenes video 1 and video 2 You might not notice the difference, but the few extra seconds really show how Callie and Erica feel :D