I would hesitate to disagree with Dave Oesterreich, but the fact that you even mention the possibility of a 4-string suggests you miht have some thoughts in that direction.And I don'tthink its by any means a ridiculous idea. there are obviously a lot of oldtime tricks you couldnt do without a 5th string, but you wouldnt be the first person who's played fiddle tunes on a tenor tuned banjo ( probably capoed or tuned up to DAEB) or in the modern irish/BritGDAE low tuning. And there is a long history as well of finger or plectrum on the DGBD tuned banjo as well. If youre interested in song backing without benefit of guitar,I'm sure you'll go down the 5-string road, but for playing tunes, or backing in a band context there might be arguments for a 4-string. As regards the head I'd go for skin, and put up with the climatic hazard: failing that, fibreskin.