I'm a fan of using the top 4 strings of a standard set. These can easily be tuned to open D - DF#Ad, or open G - DGBd. This open G tuning is great for playing a variety of chords because it is like a banjo tuning without the 5th string (you can use a banjo fingering chart). It also works well for blues as well. Switching between the two tunings is really quick and easy also.
I would highly recommend Open G tuning (Dgbd).
Have you seen my instructional video CD 5 (How to play 4 string CBG)?
Please feel free to write if you have any questions. Enjoy.
I usually prefer open E, EG#BE, then it follows the same fretting as barre chords on standard tuned 6 string, just easier for an old garage rocker like myself to transpose stuff to 4string CBG i guess. The songs on my page and the video of me on Jason Lambert's page are all done in open E.
With my 4-string tuned to DADF#, I am finding most of the chords I want using barre chords, but haven't found a C chord that sounds good in your typical GCD song. The barre chord sounds way to high to me and the C chords I find on charts don't seem quite right. Is that just a drawback of open tuning on a 4-string, particularly one using the lower four strings instead of the upper ones?