I don't believe there ever was a Trooper Mark 4 (or IV). They went from the Mark III to the Mark V. That would be an attractive price for a Mark III in good shape - I have a couple of them and they're very solidly built, and great shooters. I don't know much about the Mark V model, but I suspect that would be a good price for one of them as well.
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Pemigewasset Valley Fish & Game Club