The topic should include some kind of relationship between one or more variables and individual human behaviour (or mental processes). You must find these variables in topics, such as: Compliance, obedience, conformity, identity, attachment, addiction, depression, perception, thinking and decision making, memory, aggression, love, etc.
The topic and the variables involved will work if you can find at least 10 sources which combine those variables.
Use text books, journal articles or pdfs.
Research must become focused. Maybe at the start of the research you will find lots of information that are related but it is important to distinguish between the sources that combine your variables and those which do not. In 4000 words you must become an expert, to that you must keep the focus on the variables you are investigating on. This will also affect your effective use of time.
Your research must include: definitions of the chosen variables, theories/models as well as empirical evidence supporting the relation between your variables, factors affecting that relationship and counter-argument and/or alternatives explanations.
Keep the references of your sources. If you find something useful, make sure that you can find it later and that you can reference it.
Go with the flow. Your writing should go from the broadest to the most specific. There must be consistency in the sequence of information. It is important that the way in which the topics are presented is not only organized but also that the paragraphs are connected.
As soon as you write a paragraph, add the references. It will save you lots of time.
Read and re-read to evaluate the fluency of your paragraphs. Better if someone can read it aloud to you.
Be flexible to new ideas and new connections between your ideas. Sometimes, you find something that makes everything clearer.