hi,
is your organic traffic dropped?
you still have your
pr4. you can wait for next pr update.(toolbar is just guessing pr) but i suggest not to do that again to switch again to php.
why did you not post question here before switching it back again to html?
it was
safe to use 301 but as i told you toolbar pr is just guessing pr for internal links( since you have 301 in your header and it was treated as new by google or toolbar). you need to wait until the next update to see your real pr.
ill suggest you to re-write your index.php to index.html instead.