You shouldn't need to change any files, just some scripts you have might not be compatible with PHP5. PHP4 is getting old though, and most are phasing it out.
well, the coder that am dealing with is telling me that he must re-write all the files again to php5 :S and this will take a few months so i must know if he's right or no
The amount of work depends on the scripts in question. Some might need a complete rewrite, most should just need minor changes. It sounds like he doesn't want the job to me. But to be fair to him, cleaning up non-remarked or poorly written code would not be a job I would choose to place at the top of the list either.
Most of the things that should be updated from PHP4 to 5 is its OOP coding and depreciated functions. Most non OOP code will work fine if you aren't using any depreciated functions.
What is a link to your site. If you post a link it should be reasonably intuitive to gauge an ballpark idea of how accurate the developer is being based on what kind of pages you have.
Shouldnt really be a tough task, a common issue is deprecated functions, which have (in most cases) alternatives. Example: ereg_replace() PHP 5 Alternative: preg_replace()