You can build a four string, and put 3 strings on it until you learn, then add the fourth later (or not). Or build a 3 stringer, learn on it, and then build a four stringer. CBGs are like potato chips. You can't stop at just one.
Some may say GDgd (5431 strings)...others might suggest GDgb (5432 strings). Whatever gets the soul charged, works.
the mystery 4th string i tune in 'B' (9GDGb 32-24-16-13)- there are many configurations you could try, use thicker strings for lower tunings (-:
I just finished a 4 stringer and could not figure out the tuning. Started out with a very slack DAD variation but it jangled too much. Then tried standard guitar tuning - thought the damn neck would rip right out of her so now very happy with GDGB. Sounds very nice and not too much stress on the guitar.
In the meantime, if you have a guitar, tune it to open G (DGDGBD) and experiment with each of the string combos and see which of the four 3-string (DGD/GDG/DGB/GBD) or three 4-string (DGDG/GDGB/DGBD) combinations you like best - or find most intuitive. Then build for that. :-)
Note: Marcus's earlier mention that GDGB sounds nice is spot on. It's also a good tuning for many different and easily playable chords.