http://www.unixuser.org/~euske/doc/cdbinternals/index.html
The hash algorithm used also only produces a 32-bit key, meaning you'll be limited to 2^32 total records. Again, though, unless your data is of trivial size, that gives you considerably more room to work with than a hard 4GB limit.
Edit: doh! can't use double-asterisk for exponent on HN