Traditionaly they are tuned in 5ths like a mandolin but lower, CGDA. they were built so that people who played the tenor banjo could play the guitar without having to learn a new tuning. (back in the early part of the 20th century)
But you can tune them any way you like. If you are used to 4ths you could tune them that way, (like guitar but without the top strings) or like the high strings of a guitar.
If you want to learn to play in 5ths, it's a bit of a stretch for some things on your fingers on this large an instrument.
Lot's of people experiment with different tunings. Have fun!