Many thanks to you dulcimer veterans for sharing your knowledge!  Once again, you have given me helpful direction in my dulcimer journey.  I will heed your sage advice.

One other thing, the doubled melody string produces a different sound/tone than a single melody string. 

Why not give yourself both options.  As Dusty Turtle has pointed out, many builders are slotting the nut and bridge to allow either four equidistant string spacing or the double melody string option.  So there's no need to limit yourself to one of the options.  You can have both if you want them.

With that being said, I prefer a three string instrument for the majority of my playing.  I do, however, own a couple of four string dulcimers that are slotted for both four string equidistant spacing or four string double melody spacing.  The extra bridge and nut slots are barely noticeable.

Conventional dulcimers have three courses of strings -- melody, middle drone and bass drone.  Any one or all of those courses can have 1, 2, or even 3 strings.  They are played, and often tuned, exactly the same regardless of how many stringare in each course -- CCCGGGccc, CCGGcc, CCGccc or CcGgcc are all played the same way that CGc is played  

4-Equidistant strings are a different ball of wax.   They have 4 courses, and that 4th course adds an additional note to the  tunings and the creation of (by dulcimer standards) some 'exotic' chords that are unlike conventional chords.

I suggest you spend some time joining and reading the posts in that Four Equidstant Strings Group

Most dulcimers made nowadays have notches in the bridge and nut so that you can string them with four equidistant strings or with three courses and a double melody.  You can switch back and forth and don't have to choose!

There is a group here devoted to playing with four equidistant strings .  You might consider joining that group and perusing the discussions to see the variety of ways people tune with four strings.

--
Dusty T., Northern California
Site Moderator

As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie
updated by @dusty: 05/28/22 01:32:45AM

I'm enjoying my three string dulcimer but am considering acquiring a four string instrument (can't have too many instruments!).  Most four strings have a double string but I've seen a few instruments in which all four strings are spaced equidistantly.  Should I stick with the standard double string arrangement or go the equidistant route?  Any input greatly appreciated.