I am the proud owner of a 1928 Gibson PG-1 Plectrum guitar. I am now completely nuts about plectrums. I also have a 1931 Martin 1-17P, a 1930 Stetson cutaway plectrum (rosewood)and am expecting delivery tomorrow of a really rare 1931 Martin 1-28P (less than 20 made.)
One of my real gems is a 1930 Weissenborn koa plectrum.
Don't let your friends make fun of your 4-string. It's a real hoot for a 5-string banjo picker (Bluegrass and clawhammer)to re-learn eveything and flat-pick the whole business.
And yes, a plecrum guitar has a long scale of 26+ 1nches.
As far as rarity, Martin for instance made at least 100 6-strings for every tenor 4-string, and 100 tenors for each plectrum. They are really rare birds.
By 1932, the plectrum era was fast closing. The bands by then started using a lot more horns, and a little 4-string lady was no longer going to cut it. They needed a big archtop 6 string boomer to be heard, so plectruma are the model "T" or buggy whip of the era; time just passed them by.