Essays come in all different types and those differences can take a little while to learn. If you are accustomed to shorter papers, the prospect of writing a four page essay can be downright frightening. Consider the points listed below so that you can get a good grasp of how you should format:
The formats for academic writing are usually decided beforehand based entirely on the subject. Social sciences favor APA, Humanities tend to use MLA and so on. Once you know the subject that you are writing within, more often than not this solves your question right away.
If you have a professor who gives you the choice to select a format you should be aware of the extent to which he or she really means that. If you have all the choice in the world you can proceed with that in mind but it may not necessarily be so. Know where your boundaries are if any exist at all.
If you truly are being allowed to pick, figure out which of the styles is most suited to you personally. It may not be what others would have chosen but that is hardly the point. If given this level of freedom it is expected that you will use it.
Every topic is different in its own way. If you pick a topic that is best explored through the MLA, make sure that you use that particular style and no other, To do anything less would be to risk low grades for no good reason.
Sometimes you may find yourself very knowledgeable about a certain style but still needing assistance to use it. In such cases, you may have amassed a large quantity of resource materials. If so, try as much as possible to use the format you have already earned.
Much of this process may a bit optimistic. Few of us are every really given the option to chose how we will structure out work. The styles have certain principles and a professor of a particular faculty will have become accustomed to being handed work that is done as closely following instructions as possible.