Pick guards on four string banjos are old hold-overs from the past when frosted heads dominated the market. The were simply used to eliminate the sound of the fingers brushing on the head, which was pretty noisy on a frosted head. They were also used on many old calf-skin heads, but now they're basically obsolete. I'd bet you'd be hard pressed to find one that isn't an old survivor from the past. What you really need is the following taken from the Eagle Music website:
"SMOOTH TOP FROSTED BOTTOM: This head is popular with tenor and plectrum banjo players in that the smooth top of the head is not offensive to the brushing of the hand over the head, it is quieter when they strum their banjo and pass their hand or plectrum over the head in the process. The spray coated / frosted bottom gives good bass and more depth of tone than a clear head."
Hmmmm, are you talking about a pick guard or an armrest?
I've never seen a plectrum banjo with a pick guard, although that doesn't mean they aren't around. On the other hand, plenty of plectrum banjos have armrests, and so do most tenor banjos as well as some five-string banjos. The purpose of the armrest is to prevent your forearm from resting on the banjo head or rim and dampening the sound. The armrest is usually attached to the resonator or two of the rim hooks, and it floats slightly above the banjo rim. An armrest probably isn't as crucial to five-string players if they are using finger picks and are picking instead of strumming.
Most plectrum and tenor banjos have some scuffing on the head, in and slightly below the area where the pick hits the strings (and sometimes the head). But as far as I know, that pretty much comes "with the territory," as they say. SETH