A listener asks: "Can your lessons be adapted to 4-string tenor guitar playing if it's tuned DGBE? Looks like the first lesson in Book 1 works fine."
I play tenor banjo tuned GDAE and some CGDA, but have no experience with DGBE on a banjo.
However, as these are the same highest 4 strings tuning as a guitar, then you would be able to play any tunes that don't have notes lower than D using the same fingering as you would on the guitar.
What to do with tunes that have notes lower than D? Well, this occurs all the time with instruments such as D Flute, tin whistle, and Uillean Pipes, where the lowest note is in fact D.
What practitioners of these instruments do is extemporise, that is re-create the phrasing of those lower notes (notes that are non-existent on their instruments) and "put in something" to fit the instrument. In other words alter the melody.
The last tune in this clip is Farrell O'Gara, a tune that uses a number of low A & B notes. Played on fiddle and concertina there is no problem, as both instruments contain all the notes to the tune, including low A and Low B.
In the second clip you might be able to hear the flute player re-phrasing the tune to fit the instrument, the flute not having the Low A and Low B. In fact she does it with the next tune also, Lads Of Laoise, which contains both low B and Low A.
I guess it's worth considering that by using a DGBE as compared to a GDAE tuning you are sacrificing half an octave of notes, that is G, A, B, C and their sharps. But it's also worth noting that Gerry O'Connor uses a CGDA tuning so he doesn't have Low G, A, or B in the third clip.
Another advantage of learning GDAE tuning is that it is the same as mandolin tuning, so at some stage in the future if you decide to play mandoline then apart from some adjustment for the smaller fretboard the switch is pretty easy.
Of course there's nothing wrong with using a number of different tunings. I myself use GDAE on banjo and mandoline, occasionally CGDA on banjo, EADGBE on guitar, and also two open tunings on guitar - DADF#AD (open D) and EAC#EAE (open A).
Hope this helps, good luck with it.