In addition to the above, you will want to ensure you can update your server to a compatible version of PHP. On this upgrade, you are likely to need to upload a fresh set of files and upgrade from /admin/upgrade. The reason for this is you are going to need to update to PHP 7.4 to upgrade, and Im not sure 4.3 will work on that version.
In general
Make sure you can obtain newer versions of any plugins/applications that are needed
Make sure you can obtain newer versions of your theme
Make sure your server can meet the latest minimum requirements
Test if you can do so first
Most importantly - Ensure you have full backups of both files and database, and that you can restore them to fully working order if something should go wrong.