3
It might be that your server is hacked.
Take a backup and try to compare your files with a backup from a previous unsuspected time. Especially the files that are most hit.
Manytimes index.* files are modified and contain added or other than the standard scripts.
Quote:
5025 / for the index page
1087 /404.html error page
1058 /modules/system/admin.php
These figures are for one day?
The 404 errors means some files are missing or request have been made with an nonexistant file name.
Get your logs and sort by IP. Take the ones that involve the most traffic and especially the ones that do POST requests. Look up these IP's with whois and deny access for the entire net they belong to.
Provide a 403.html file to return as error file for forbidden with a length of 0 bytes.
If you got really 5000 uniques then another possibility is that your site contains controversial or extrordinary material and you got slashed. In this case you would see the same referral over and over. You could ask them to remove the link to your site.
And what was you former nick?