Four string spacing pros and cons?
General mountain dulcimer or music discussions
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.