Hey,
This depends on a few things, namely the weighting of the test and how good your cohort is. If the weighting of the test is low (like 20%), then you still have the chance to get a high rank. However, if the weighting is high, then you need to work harder at acing trials and externals. This test might still disadvantage you in terms of rank, but I don't think it will prevent you from achieving a good mark, as long as you work hard for your other assessments.
Now achieving a band 4 or 5 somewhat depends on your cohort. If your cohort does well, then your assessment mark will be lifted, and if you do well, you can achieve a great mark. If your cohort doesn't perform as expected, then your assessment mark might get dragged down.
If I were you, I would do two things. The first would be to determine why you didn't do so well in this test. Did you not have enough time to finish the paper? Do you need to go over your understanding of concepts again? Questions like these will help you to "diagnose" the problem that is setting you back from achieving your best. Once you have done so, figure out a solution to these problems, and make sure they don't happen again when externals and trials arrive.
The second would be to help my cohort. It makes sense, doesn't? Not only are you helping them in getting a better mark, but also you are enabling yourself to get a better mark! The better your cohort scores in the external, the more your assessment mark will be lifted up. It's a win-win situation.
I hope that helps! Obviously, I can't guarantee whether you will get a band 4 or band 5, but if you work hard (and smart) now, you will be able to achieve a good mark!