Make sure that you keep up on your committee meetings. It is really crucial in these years. You need to be making sure that your are making steady progress on your project, and if you aren't, then you need committee input to help you and your advisor to plan a more productive course.

As you accumulate data, put it in final figure form. You will never have a better grasp of the data and how you generated it than you do at the time, and it is really painful to reconstruct the details later. This is the most time-consuming part of writing your thesis. If you do it as you go along, it's pretty straightforward to produce a good thesis.

By this time, you would hope to be at least part of a publication. If so, get involved in the writing, in the presentation of your data in figures and the final preparation of the manuscript. This is a skill you need! Writing your work with publication in mind makes your thesis writing straightforward, and you need publications when you finish, whatever you decide to do after your Ph.D.

By the middle of the 4th year, you need to be looking ahead to your next step -- a post-doc, a job in industry, a change to another application of your degree? Talk to your advisor, your committee, anyone else who might be helpful -- but begin to make plans!