The problem is the scale, or the length of the neck. A five-string with the fifth string removed becomes a plectrum banjo, and it is still tuned either in open G or in DGBE, Chicago tuning. A tenor with 19 or 17 frets (as opposed to the 23 on a regular banjo) can take greater string tension and allows you to tune a banjo either CGDA or GDAE. This is known as fifths tuning. If you want to play dixieland, you can just play with the fifth string off. But to play Irish styles or other styles you need the shorter scale length of a tenor.