Pony Kegerator

We would like to have two different beers on tap with two beers back-up for the change was easy. If you were to keep the outside of CO2 per barrel could you accommodate 4 thin barrels (7.75 gallons) in a drum standard (6.4 ft)? Or would you need a special barrel for it? Where do I look?

You can put two 1/4bbls thin, not four.
You can install 3, sometimes 4, depending on the inside of the unit, barrels 1/6bbl (5.16gal) which are the size of most craft brewers use rather than ponies. You can get better beer in this way as well!
Cheers!

Just an idea for a project with a second hand refer cheaper than Kegerator.
http://www.kegkits.com/kegerator2.htm
http://www.beveragefactory.com/draftbeer ...
Corny drums fit in well in the reference converted. Me know I have converted two taps.

Kegerator Dual Faucet Two Product
http://www.beveragefactory.com/draftbeer ...

With extensive research, I am sure you can find a Kegerator that fits your needs. But at what price?
Good luck