I've got these two:
http://www.performancebike.com/shop/...slisearch=true
This one is tiny, takes threaded cartridges, and works well. It starts flowing CO2 when you push down on the head.
http://www.nashbar.com/profile.cfm?c...t:%20Inflators
I like this one slightly more because it has a valve to control the flow.
If it's only 4 miles carry two spare CO2 cartridges and if you get a third flat, take it as a sign to walk or call for a ride!!!
Look on Ebay for threaded cartridges. I got military grade 16g cartriges for less than a buck a piece.