Compatibility of mysql 5 with mysql 4

Are there any known issues regarding the upgrade of mysql from 4 to 5? I mean are there any known scripts which may face problem with mysql 5?
I have a dedicated server with mysql 4.1.22 and I am looking to upgrade to mysql 5 but I worry for my clients hosted. Some of them are using very old scripts such as oscommerce, phpbb, yabb, etc.

 

 

 

 

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