We (my fellow dulcimer players and I) have always considered a dulcimer with a doubled melody string to be a three-string instrument, and four-string only when you're playing with equidistant strings. Maybe not technically correct, but you're still only getting one string's worth of notes out of a doubled melody string. Oh, well. I have two dulcimers and neither has notches for equidistant playing, but my experience has been that that's mostly used for fingerpicking and I don't do that much.

I've had to get books at festivals and specialty music stores. There are probably some online, too. Most of what I have I've gotten through dulcimer club, though. I don't know where you live, but if there's a club in the area, they could be a big help.