Jason--Thanks, and that's not necessarily good news at all. That's an appreciable distance above the cascades, farther than I would have thought it likely for "bucket biology" rainbows to have migrated when there's plenty of fine water lower down. On the flip side of the coin, I don't think a bucket biologist would do illegal stocking while making that long a trip or, for that matter, that trout would survive after that long a hike.
A bit more heartening is that there were only four of them. How long a section of stream did you survey. If there were four in 50 meters or so, that's a dark cloud on the Lynn Camp horizon, but if the area covered was much longer, it's a bit more heartening.
Did your group find only rainbows or were there specks? If there were no specks, there's clearly reason for significant concern.
Thanks for your fine report (and your volunteer work with the Park).