Season 4

Season 4 began December 7, 2008 and ended October 31, 2010. Timewise, it's the longest season of Robot Chicken. 48 episodes were aired.
In April 2010, the show took a five-month hiatus that lasted until early October 2010. This hiatus was due NM to time needed to produce Season 5 and the Star Wars special.
The episodes in Season 4 spell out secret messages/historical events/scientific theorems
The first message:
Episode 1: Help Me (12/7/08)
Episode 3: I'm Trapped (12/21/08)
Episode 4: In a DVD Factory (12/28/08)
Episode 2: They Took My Thumbs (12/21/08)
Episode 8: Two Weeks Without Food (1/25/09)
Episode 5: Tell My Mom (1/4/09)
Episode 10: I Love Her (2/8/09)
Episode 9: But Not in That Way (2/1/09)
Episode 7: Love, Maurice (1/18/09)
Episode 6: P.S. Yes in That Way (1/11/09)
Second message:
Episode 25: Dear Consumer (12/6/09)
Episode 17: We Are A Humble Factory (7/26/09)
Episode 18: Maurice Was Caught (8/2/09)
Episode 19: Unionizing Our Labor (8/9/09)
Episode 20: President Hu Forbids It (8/16/09
Episode 21: Due To Constraints of Time and Budget (8/23/09)
Episode 22: The Ramblings of Maurice (8/30/09)
Episode 23: Cannot Be Erased, So Sorry (9/6/09)
Episode 24: Please Do Not Notify Our Contractors (9/13/09)
Handicap Theorem:
Episode 11: Do The Handicapped Go To Hell? (3/22/09)
Episode 12: Probably (3/29/09)
Episode 13: Professor Chaos (4/5/09)
Episode 14: Simpsons Already Did It! (4/12/09)
Bargain Slogans:
Episode 15: Reunited and It Feels No Good (4/19/09)
Episode 16: Not Without My Cash Cow! (4/26/09)
Misc.:
Episode 26: Go God Go (1/3/10)
Episode 27: To Kill A Mockingbird (1/31/10)
Episode 37: Son of Stan (4/18/10)
Episode 28: 200 (2/7/10)
Episode 34: 201 (3/28/10)
Episode 29: Coon vs. Coon and Friends (2/21/10)
Episode 31: Coon 2: Hindsight (3/7/10)
Episode 32: You're Getting Old (3/14/10)
Episode 33: Ass Burgers (3/21/10)
Episode 35: Cat Orgy (4/4/10)
Episode 36: Jewbilee (4/11/10)
Episode 38: Two Guys Naked In A Hot Tub (4/25/10)
Episode 30: Mysterion Rises (2/28/10)
Misc. 2:
Episode 39: Go God Go XII (10/3/10)
Episode 41: Pins, Spins, and Bins (10/17/10)
Episode 42: California Dreamin' (10/24/10)
Episode 40: When A Man Loves His Cookie (10/10/10)
Episode 43: Especially the Animal Keith Crofford! (10/31/10)
Help Me
July 11, 2004
They Took My Thumbs
July 18, 2004
I'm Trapped
July 25, 2004
In A DVD Factory
September 5, 2004
Tell My Mom
September 12, 2004
P.S. Yes in That Way
September 19, 2004
Love, Maurice
September 26, 2004
Two Weeks Without Food
January 23, 2005
But Not in That Way
January 30, 2005
I Love Her
March 6, 2005
We Are A Humble Factory
May 29, 2005
Maurice Was Caught
October 2, 2005
Unionizing Our Labor
October 9, 2005
President Hu Forbids It
January 15, 2006
Due To Constraints of Time and Budget
January 22, 2006
The Ramblings of Maurice
January 29, 2006
Cannot Be Erased So Sorry
May 14, 2006
Please Do Not Notify Our Contractors
July 16, 2006
Especially The Animal Keith Crofford!
November 12, 2006
Dear Consumer 
November 19, 2006
Alternate episodes:
For the Main Bathroom (Airing February 2, 2014)