Trouble updating plugins, Uploading images, updating WP

Hi,
I have been trying to update plugins and wp for a week now- reading forum posts and more. I am also having trouble uploading a pdf- I get a HTTP Error.
I have tried the regular uploader and flash uploader
I have changed the .htaccess file (and changed it back)

I have spoken to the host- they use php4 not php5 (could this be the issue?)

I’m pulling my hair out trying to understand what is going on with this site
I have a number of other sites that all work fine- all hosted with JustHost- it’s just this one site hosted elsewhere that is causing problems

Any clues anyone?

Thanks for your time,

Phil