A team would be penalized for not getting the clue sending them to the U-Turn location, or by following another team without finishing the task first. Depending on the circumstances, they might receive the penalty for not completing the Detour, and for not having the clue if they get to the pit stop without all the clues in hand to check in. In the past, Phil has sent teams back to get the missing clue. So I'm not sure exactly what would be done in an instance with two significant rule violations, but 4 hours for the one, and probably at least another half hour plus time saved or whatever it would be to neutralize the impact on the U-Turned team would be a good guess, I would think.
The closest I can think of what the Momily penalty in the first Thailand leg of TAR Classic, where they quit the detour, did not have the clue to the next location, and arrived at the pit stop having not gotten all the clues. All we heard was a 24 hour penalty. I'm starting to wonder if the 24 hour penalty then was for multiple violations and not just quitting the detour; since we never saw a quit the detour violation again until very recently, and we were all surprised it was only 4 hours.