I'm working on arm and leg hair for M4 in the hair room. I've got a pretty decent right arm brewed up, so far. Is there a simple way to mirror this over to the left arm? I've searched messages here as well as the user manual, and don't see a thing that looks helpful.

I've yet to mirror anything in the hair room. The closest I've gotten was once I was working on a hair group that spanned the whole head and when I selected strands on one side I accidently selectd them on the other too so I only did half the work. But that won't help you. If you grew the hair on a prop and have p10 or pp2014, you might be able to copy that  and use the fitting tools on the morph brush to make that prop fit the other arm but otherwise, I think you'll need to do it from scratch. There aren't many shortcuts for the hair room yet.

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I use Poser 13 and win 10

Having the same problem.

If this is for your own use think a solution could be to create the scalp from the base mesh in a third party app that does support symmetry and import into Poser as a prop (and grow the hair on the prop as RedPhantom has already pointed out).

Sounds a lot of effort but once the two apps are running with the scenes loaded its easy to make adjustments and add other groups as required.