It's hard to know whether this is a good timeline without knowing some more information about you. If time weren't a factor at all, the best time to take the GMAT is when you're consistently scoring at or above your target score. Since you (like many students) have deadlines to consider, you can't necessarily be that flexible.

So, have you taken practice tests? And how close are you to your target score? How much time are you allocating to studying each day? If you already have a solid understanding/mastery of the concepts, 3-4 weeks should be plenty of time to refine your skills and be ready for the test. If you're still in the process of learning concepts, though, you might want to push it back a week or two.

It's always wise to plan for the possibility of taking the GMAT twice (you might get sick or have an off-day the first time). That said, many schools will allow you to take the GMAT directly before the application deadline (without waiting for the official score report), so you may be able to push it back as far as April 30. See what the schools you're applying to require.

Good luck!