Consider what you can accomplish in four hours: perhaps 50 practice quantitative questions, including time spent reviewing and redoing them to ensure you’re learning. Perhaps 15 reading comprehension passages, 30 critical reasoning questions, or 50 sentence correction questions, all with time left over to analyze how you did and read explanations.
Sure, when you take a practice test, you’ll get plenty of practice questions, but you aren’t learning very much from them. You’re trying to work through them quickly, perhaps skipping some, and then by the time you’re done, the last thing you want to do is go through them again and review the ones you missed. And in some cases (including the official GMATPrep tests), explanations aren’t even provided.
As I say, the benefit is great a couple of times: you need to know what it feels like to work against the clock and to skip questions when necessary. But the most important aspect of your preparation is always to learn as much content as you can possibly apply to the test within a reasonable time-frame. Working through and reviewing practice questions does that. Taking a fourth or fifth practice test does not.