My father-in-law had a 12 string mandolin-- instead of two strings for each tuning it had triples. I have never seen another like it and I'm sure it took strong fingers to press three strings at a time. Unfortunately it was stolen in a house burglary.
my [late] husband used to have a beautiful 12 string guitar. we had to end up selling it and years later when our sons started playing music, it was so sad that it was gone. all troubles fade away when you hear a 12 string played by someone who knows how to play... it even had a blue suede gultar case. the guitar in Stairway to Heaven is played on a 12 string. and it's such a gorgeous sound.
Four string guitars are sometimes called tenor guitars, they have the bottom four strings. The fingering is very much like a uke or a four string banjo CGDA. The neck is slimmer than the 6 string and the body is usually a little smaller. Nick Renolds of the Kingston Trio played one. (I have one too.)
it helps to have a keyboard or a tuning fork. if you can tune the low E string first, then it can be done with a good ear.. although I've heard my son get pretty frustrated at times tuning his bass. both of them are musicians, so I've picked up some of this.