Have you tuned open D (DAd) on a three string? If so, just add the next smaller string onto whatever you used in the past. Tune that string to f#.
Otherwise, what is the lenght of your scale?
If you're fretting the 4 string, an alternate "D" tuning is ADF#B, which is one version of ukulele tuning. Difference is with a longer scale length, as with a guitar (24-25.5 inch), it will sound like a guitar, and not a uke.
Use the 5432 strings from a standard set of guitar strings (ADGB) and drop the tuning on the G to an F#.
Lots of chord charts to be found free online for the client to learn from too.
Scale length is 26". This is the scale I used on the dulcimers I built and it was easier to just transfer this to CBGs than to work up a new pattern, so I took the easy way.
I haven't had much luck working with DAdf#, but I haven't given up on it.
Closer to standard guitar open tuning is DF#Ad. If you tune to Dm by flatting the F# to F then it is a really dark sound that I like a lot. You can still do the boogie thing, play turnarounds and whatnot with out the 3rd scale note getting in the way on the high string.
There are many, many possibilities. I suggest maybe doing the guitar with Dadf# which a lot of people use and play and supplying an extra pack of strings with some other ideas on tuning the customer may wish to try.
Thanks, everyone for the info. I'll get it together with info to pass on to the customer.
Appreciate all the help.
theres also (DAda) tuning i use a 6th E tuned down to D 5th A tuned A 4th G tuned down D 3rd B tuned up to A its a low tuning but its close to (DAd) on a 3 string i find it useful and spooky heres a vid of me in (DAda) tuning
The open "D" tuning used on most 4-string guitars (D-F#-A-D) is based on the open "D" tuning of 6-string guitars (D-A-D-F#-A-D), so you can use strings 1-4 of a standard set of strings for a guitar. In my experience, coming from a 3-string diatonic stick dulcimer background, and having now built several 4-string chromatic instruments, D-F#-A-D tuning has proven to be a good alternative to the more traditional A-D-A-D tuning that I began with on 4-stringers. Stick dulcimer players often play the melody up and down string 1 (the "melody string") while just strumming the other strings (the "drone strings") without fretting them. On a 4-stringer, you can play your old tunes (melodies) on string 1 and with the additional string, have three strings for the drone strings, producing a more complex sound. The D-F#-A-D tuning provides a different drone sound which I like. And I believe, that by having one of the drone string tuned to F#, I should be able to find more chord possibilities than just having A-D-A-D tuning. You can locate chord charts for open D tuning on the six string guitars on the Internet, and just ignore the 2 missing strings to come up with chord charts for D-F#-A-D tuning. In fact I bet someone on CBN has already gone that work. [Anyone have that URL?] As of yet, I haven't made the transition to chordal music, but hope to do so sooner than later. My latest 4-stringer uses gold fret wire for the familiar "natural" notes and silver fret wire for the "accidentals", so I can more easily learn to navigate the fretboard in my transition from 3-string diatonic instruments to 4 string chromatic instruments. I believe learning D-F#-A-D will go a long way in helping me make this transition. Here's what I'm starting to practice with for chords:
Thanks Rand, Lots of great info, the chord chart are great as well. A lot of help.