Tips to Learn 4-Eq. Strings
I'd like to open a new discussion of how people learn or teach 4-eq. so we can help others. I hope you all will chime in here!
I would recommend you start out by adapting your current tuning across the four strings. If you play in DAd, and especially if you have a double melody string, it's easy to turn that into DAdd by separating those two melody strings. You would then play on the bass, middle and outer melody string and let the inner D string drone. This way you can use familiar chords and fingering you already know.
The drone will give you some beautiful chords, although at times you might notice a dissonance, where notes don't seem to sound good together. You can practice pressing the inner string along with your melody string to get around that, or just go with the flow. I got so I liked the D droning almost all the time. The times I'm most careful to get rid of the dissonance are when I'm playing pieces from, say, the 1600's or certain old-timey tunes. You can also press the inner string so it gives you an additional harmony or adds another note to a chord. But for starting out, I'd suggest you let it be till you get your bearings.