I'm writing a four page research proposal as a reply to a call for proposals for funding. Should my document be organized by sections ? or it should only contain text ?
As user rhialto suggests, it may be field dependent. But I'd suggest that you look at it like this:
The proposal needs to be easily readable and it needs to be clear that you have addressed each required element mentioned in the call. Normally there will be several such requirements. So, organize the proposal according to those requirements with a separate (titled) section for each important element. These might include such things as Researcher background and Work to be done.
Make it so that in a committee meeting, where proposals are discussed anyone on the committee can easily find the important elements and how your proposal fits them.
In a four page proposal, if there are a lot of requirements, you might have to combine a few. And don't format it so that it is too much dominated by white-space, though some white-space and reasonable font sizes is a help. Find better words, not just more words to get the ideas across.
Good scientific writing isn't just about having an idea worth putting on paper, it's also about conveying that idea in an understandable and unambiguous manner. A research proposal will usually have several distinct, but related aspects, including problem background and previous work, the hypothesis you want to test, and the methodology you're proposing. Breaking your proposal into sections will improve readability and allow your readers to focus their attention. The only downside I can think of is the minor loss of space if your page limits are very tight. Particularly when your writing is read in a competitive context, where you are competing for limited funding, you want to remove every impediment possible to the committee understanding your work.