For such a small project, as little as possible. What types of projects are you managing? What method or approach are you following?

For IT projects, especially software projects, Construx Software Builders offers a lot of useful templates and checklists, including "Lite" templates for smaller projects.

If this applies to your case, check out their CXOne templates at link text. The documents and other resources are free, but it does require that you register to get a login id.

Small does not necessarily mean simple. Your PMP needs to support the rigor and formality the project requires and it is a judgment call. There is no one right answer. This is where expertise and intuition of a seasoned PM comes into play. I think the one wrong answer is to assume a PMP is not required or staying silent on one or more of the PM areas. All projects should have all of those PM areas addressed to some degree, even if it's not more than a paragraph.

I agree with the idea of creating a "lite" project plan template. You should decide for you self what your company needs in the template.

You might be able to group small projects into categories (e.g. projects with similar risks, projects that affect similar areas of the company, etc) and have project plan templates for each category.

Assess the complexity of the projects to determine how much project management you need. Four people on a six month project might be able to self-manage, but if they are all multi-tasking on different projects of varying priority, they will likely need someone coordinating.

I think, no matter how small, a project always needs objectives, stakeholders identified, milestones & progress checkpoints (i.e. a schedule), prioritization and probably more... but generalizations like that won't help you too much; you have to spend some time assessing each project.

What should it contain? As little as possible.

What is the project, what are the requirements (scope), what are the desired outcomes, who's involved (stakeholders), when does it need to be done by (schedule), what is the budget (cost), what do those 4 team members need to complete the project?

And of all of those, which is the most important? Cost, time, or desired outcomes?

Beyond those simple (but key) questions, you only add things as project complexity (or company requirements) warrant it.