Official CBS Synopsis:
HOW I MET YOUR MOTHER is a comedy about Ted (Josh Radnor) and how he fell in love. It all started when Ted's best friend, Marshall (Jason Segel), dropped the bombshell that he was going to propose to his longtime girlfriend, Lily (Alyson Hannigan), a kindergarten teacher. At that moment, Ted realized that he had better get a move on if he, too, hopes to find true love. Helping him in his quest is his friend Barney (Neil Patrick Harris), a confirmed bachelor with endless, sometimes outrageous opinions, a penchant for suits and a foolproof way to meet women. When Ted meets Robin (Cobie Smulders), he's sure it's love at first sight, but destiny has something else in store. The series is narrated through flashbacks from the future.

How I Met Your Mother (or HIMYM) has one of the best running gags ever: the slap bet. The important things to know are that Barney lost a slap bet to Marshall. He was given a choice, and he chose five slaps to be delivered whenever Marshall wanted. There are two left: the first was in "Slap Bet," the second in "Stuff" and the third in "Slapsgiving."

Season 4

1. 4.01 "Do I Know You?" (9/22/2008)
2. 4.02 "The Best Burger in New York" (9/29/2008)
3. 4.03 "I Heart NJ" (10/6/2008)
4. 4.04 "Intervention" (10/13/2008)
5. 4.05 "Shelter Island" (10/20/2008)
6. 4.06 "Happily Ever After" (11/3/2008)
7. 4.07 "Not a Father's Day" (11/10/2008)
8. 4.08 Woooo! (11/17/2008)

Neil Patrick Harris as Barney
Jason Segel as Marshall
Alyson Hannigan as Lily
Josh Radnor as Ted
Cobie Smulders as Robin