If you want to play bluegrass banjo, or old time for that matter, you pretty much need the 5th string in order to syncopate the finger picking or frailing respectively. Flat picking guitar style is what is played in bluegrass and is typically done on a standard tuned acoustic guitar, you could do this on a gitjo but it won't be able to directly replace the role of the banjo in a bluegrass band. If you already finger pick guitar you could try and learn banjo rolls and songs on a standard tuned gitjo but if you want to play a banjo song as written for the banjo it would be pretty challenging to memorize because the 5th string on the banjo is on top in order to use your thumb on it because of how often you use it. To get that sound you'd have to be picking the high e as often which is opposite the other strings, doing so with finger picks on (pretty much required to get bluegrass banjo sound) seems harder than just taking a little time to learn how to play bluegrass music on the instrument most often associated with it; bluegrass sounds the way it does because of the way a 5 string banjo is designed. As far as chords go, it's pretty easy since the strings are not wound and can be barred with one finger, and each barred fret gives you a major chord. If you are already a comfortable guitar player I'm sure you would pick up the 5 string banjo quickly with a couple free youtube tutorials or an entry level book.