Well you can convert a 3 String CBG into a 4 string by either adding a 4th string to the existing neck (which may require making some extra mods to it) or by replacing the neck. I'm planning on converting my 4 String Cigar Box Guitar into a 6 String (maybe a 7 String) by making a new neck for it. However it'll be wearing Nylon strings to keep the tension down & tuned to Open G (but down a Half step so Open G flat). I also wanted a Russian 7 String Guitar but they're very overpriced.
it all depends on how the 4 strings are tuned. you could tune it GDgd then just use the GDg and ignore/mute the d string. If you wanted to use all the strings then just play the same fret on the d as you are on the D string it will just add an octave note. Shane sometimes uses other tunings than GDg .
You'll really need a Special Conversion kit. The next idea I had was to convert my 4 String Cigar Box Guitar into a 6 (or maybe a 7) String cause I don't want to built a new instrument & get another Cigar Box, I wanted to simply swap the neck
Your 3 string guitar is usually tuned ( 1 - 5 - 1 )
The 1 is the root note of your chord and the 5 is the 5th note of the chord and the other 1 is the root note at a higher octave.
The 4 string is usually tuned ( 1 - 5 - 1 - 3 ).
Your choices are:
A: Keep the 3rd note string of your 4 string guitar tuned as is and add that note to the chord you are playing.
B: Mute the 3rd note string.
C: Tune the 3rd note string to a 5th note at a higher octive than the other 5th for a fuller 3 string tone.
I think C you'll get a much fuller sound. Well we could have a 7 String version tuned DGBDGBD like a Russian 7 String Guitar cause whatever yo do on the bottom 4 strings can be done on the top 4 as well. Converting a 4 string Cigar Box Guitar to support more strings might require making a new neck.
I made a Baritone conversion neck and put it on a Strat. It has Dulcimer fretting, I call it the Ducitar. 6 strings at 28" scale.
It's tuned open C : C/G/C/B/C/C. You can play Bass chords on the 3 top - lower tuned strings, Baritone Guitar on the 4 bottom - high tuned strings or just fret the 2 - C strings while strumming the others for Dulcimer.
Takes awhile to get used to it, but it's fun.