Currently I have an existing MySQL database server using v4.
Due to some security reason, I need to upgrade it to v5.
However, as the data are important data, I need to check if the upgrade will cause me to lost my data, before proceeding.

Can someone advice me as to whether upgrading my MySQL database server from v4 to v5 will cause any data integrity problem or any other data problem please.
Also, is v5 backward compatible? Must I change my program as well?

Hope someone can advice regarding the above.

Thank you very much.