Short version--from my perspective, the research experience essay is to make sure you're interviewed by people with interests similar to your own and so that the interviewer isn't completely clueless coming in. Unless you come off as a complete idiot in the essay, it is unlikely to play a role in any admission decision.
Longer version:
There's a lot to review in an applicant's file. Especially if I'm in a rush before the interview (it's rare in my experience for interviewers to get the application earlier than the afternoon of the day before the interview), there's no way I'm going to read a 4 page essay on your research experience--a page is about the limit to do any more than a cursory glance. And as I student interviewer, I'm likely to spend much more time reviewing your application than a faculty interviewer will.
I just want to get a basic feel for what you've done, and maybe do a quick Google search so I know something about the subject if it's not my area (not looking at papers from the lab, more like looking at the Wikipedia page for the protein they work with).
In my opinion, for the essay you should be able to condense each research experience down to a paragraph--what were the goals of the project, how you went about it, brief summary of results. If one of the projects piques an interviewer's interest, they will ask you about it in the interview.