Which research question would be most appropriate for a four- to five-page research paper? what causes childhood obesity? should the government impose restrictions on what kinds of foods can be served in school cafeterias? how do french fries affect the brain? as of last year, how many obese adults in the us were also once obese children?

Generally speaking, these topics would make a great single-paged essay. However, we need the queston (presumably the headline) that will have enough information to cover 4-5 pages of detail.

"How do French Fries affect the brain?"
This is a good question, but will probably only get 2-3 pages complete at maximum due to it lacking in variety.

"As of last year, how many obese adults in the US were also once obese children?"
This is also a good question, but only covers a year's worth of work, which will lack in detail to suffice for 4-5 pages.

"What Causes Childhood Obesity?"
This question is excellent for a 4-5 page research paper. It has a variety of reasons to answer, and it can provide explanations with side-notes for each branch of information that connects to the factors of childhood obesity.

Your answer is "What Causes Childhood Obesity?"

I hope this helps!