I was never taught the 5-paragraph essay as a form. I think the rigidity of the form is overemphasized in basic instruction, because many young students think essays have to have 5 paragraphs. That's really the shortest a complete well-organized essay can be, with an intro, conclusion, and three paragraphs in the body. With four paragraphs, the two interior paragraphs are just as long as the intro and conclusion put together, so that doesn't quite work out.
Yet in a longer essay it is often helpful to write shorter sections of 4-7 paragraphs, each developing a separate sub-point. If you had five such sections, you would be essentially reproducing that structure on a larger scale: an introduction (3 - 4 paragraphs), three interior sections of 6 to 7 paragraphs a piece, and then a conclusion of a few more paragraphs.