Hi Mara,
According to an advice e-booklet available on jobs.ac.uk, it shouldn't be more than four pages, but another article on the same site says it is often many pages. I think 4 is good!
I just spent a whole day - 9 to 5.30 with no break - working on a job application, and I'd already filled in some of the basic info on the application form. Is that normal?? So glad I didn't leave it till the last minute (Monday).