i see the word ‘compensation’ used in many of the responses (as well as in the OP), which is fine. But if we’re getting into the semantics, compensation isn’t ‘due’ or required, but almost always, if you write in and give a reasoned, brief explanation, UA will provide a gesture of goodwill. At 4 hours, it’s sometimes automatic (ie, there will be an e-mail when you land with a link for options). But if it’s over 2 hours or so and not due to weather, IMO, it’s still reasonable to write in and ask, and they’ll often give some sort of voucher or miles. Not required, of course, but often, they will do it, even if you don’t ask.

sometimes, even if you write in with a more minor issue and specifically say ‘don’t give me anything’, they will. For example, a few years ago, when there were still different versions of the 739, I was on an already delayed and full flight (short delay - probably 30-60 minutes), which then was further delayed by 30 minutes or so because they assigned seats in rows 14 and 15 to passengers, even though those rows didn’t exist on the configuration of our plane (interestingly enough, it wasn’t a last minute swap, they were somehow just using a wrong seatmap). So that took time to sort out. I wrote a short note afterwards saying something along the lines of ‘don’t know how this happened, but really, this was completely avoidable, and please make sure you fix it, and I don’t want anything except the knowing that this same avoidable issue won’t affect another plane load of passengers.’ Sure enough, with my response came an ETC, even though I specifically asked not for anything.