There are many possibilities for stringing and tuning CBGs.
On a 3 string you can use standard strings ADg and tune GDg.
On your 4 string you can simply extend the range by adding the b string and tune GDgb, or add the lower E string and tune DGdg.
On my instructional video CDs (www.ebay.com Search: keni lee), I take this approach.
I take the full 6 string Open G tuning (DGDgbd) and basically cut it in half.
CD 4: 3 string (GDg) and CD 5: 4 string (Dgbd)
Due to the different string arrangements, the playing approaches are different on the CBGs, but if you then explore 6 string too, you will understand the full fingerboard.
I hope this answers your question. Please feel free to write if you have any further questions. Enjoy your practice, Keni Lee
It isn't. Follow Keni's advice - it's easier if you think about the two tunings/stringings as two slices of one bigger pattern/tuning.
I'm switching constantly between 2,3,4 and 6 stringers (actually only two "patterns" Open-G and "standard/tenor/gitarron")- EA, GDg, DGDGbd, EADG, ADGCEA). Am not a greatest musician myself, but after a while one just gets used to the different configurations.
I like 4 stringers for the reasons Keni states (flexibility in essence). Depending on the strings you use and tuning, you can easily play with 3 of the 4 strings or use all 4. Many times, I only use 3 of the 4. I use the DGBe strings and tune the same BUT know I can tune DGBd or just play the DGB strings muting the 'e' string.
If I want I can also use the ADGB strings and tune like Keni suggests.