Several things can cause a drop in page rank of the homepage
-- loss of links to the homepage or any part of the site
-- important sites that link to you loose some of their links and thus have less pagerank to pass on to you
-- changes in the links that important sites have up to you - changing the code so that they do not pass pagerank - these could be connections through a script
-- changes in the linkage structure of your site that change the flow of pagerank through it
-- periodically google recalibrates pagerank (If they didn't tons of sites would be making it to ten. This is a math adjustment similar to adjusting for inflation. If your site gains PR at a lower rate than the web average your pagerank could go down
-- adding a ton of pages to your site could cause a drop in pagerank as pagerank flows out of your main site and into those pages - similar to adding a new wing onto a swimming pool and then connecting them to allow water to flow in
-- (some people will disagree with this one -- adding a lot of links to your site that connect to other websites
-- doing something naughty that makes google drop your pagerank as a warning or as a punishment
-- if your site just recently earned four, maybe it was a very low four, now your site is showing as a very high three, this could just be flux in the calculations and rounding... keep earning links and four will return soon