I've always assumed that the Q were truly four-dimensional (or higher) beings, in that they could move through time as easily as we move through space. But Q's actions always seem to be in chronological order- that is, he talks about events that have happened in the past and seems to have no knowledge of the future (in contrast with The Prophets). But time travel happens all the time in Star Trek- they even do it themselves in Death Wish, so I find it difficult to believe they wouldn't be four-dimensional. An interesting thing to note is that Q Junior was born during the time Voyager was in the Delta Quadrant, and yet was an adult (although let's not assume anything about their lifecycles). So maybe only part of their existence is linear, the part we see.
My question is, why do the Q seem to have a linear existence? Could it be that they enforce it on themselves to exist primarily in one time? Or is there no in-universe justification, that it's just to make things make sense for the viewer?