[Quest] Four Chords

Okay so in this video of the Axis of Awesome, the pianist says that a lot of popular songs use the same four chords. I'm trying to figure out what those four chords are. If anyone can help me with this quest, then I would greatly appreciate it. Once I can figure out the four chords I'm going on the second part of the quest, which is record a medley of as many songs using the four chords as possible. I have all the information I know so far after the page break.

EDIT: Jake gave me the answer. It's D, G, Bm, A.

Information I know so far:

• In the beginning of the video it shows the roman numerals: I V vi IV. This is the chord progression used for the four chords. So by figuring out I, the other chords can easily be found. 
• For anyone that doesn't understand what I'm talking about, all songs have a key signature. The key of C major has C-D-E-F-G-A-B-C (or in solfège it is do re mi fa sol la ti do). Each of these notes can be assigned a roman numeral, and is a chord instead of a note. Uppercase roman numeral is major, lowercase is minor. In the case of C major, there would be I ii iii IV V vi vii^o I. So if the four chords are in the key of C major, then the chords would be C, G, Am, and F.
•  List of songs played (by knowing this, we can look up the key for each song and find out what it is):
• James Blunt- You're Beautiful
• Black Eyed Peas- Where is the Love
• Alphaville- Forever Young
• Jason Mraz- I'm Yours
• Train- Soul Sister
• The Calling- Wherever You Go
• Elton John- Can you feel the love
• Akon- Don't matter
• John Denver- Country roads
• Lady gaga- paparazzi
• U2- With or without you
• The last goodnight- Pictures of you
• Maroon 5- She will be loved
• The Beatles- Let it Be
• Bob Marley- No Woman No Cry
• Marcy Playground- Sex and Candy
• Men at work- Land down under
• Americas funniest home videos theme
• Jack Johnson- Taylor
• Spice girls- 2 become 1
• Aha- take on me
• Toto- Africa
• Kelly Clarkson- Behind these hazel eyes
• Jason Derulo-in my head
• Smashing pumpkins- bullet
• Joan Osborne- One of us
• Avril Lavigne- Complicated
• The Offspring- self esteem
• The Offspring- you're gonna go far kid
• Akon- So damn beautiful
• Timbaland- Apologize
• Eminem- Love the way you lie
• bon Jovi- its my life
• Lady gaga- pokerface
• Aqua- Barbie girl
• RHCP- Otherside
• Double rainbow all across the sky song
• MGMT- Kids
• Andrea Bocelli- time to say goodbye
• Robert Burns- Auld Lang Syne
• The Axis of Awesome- Birdplane
• Missy Higgins- Scar