Download only from this site
Only use the mirrors listed on the downloads page. A link passed to you in a private message is not one of them, however official it looks.
Let the launcher patch
Always start the game from the S4 APEX launcher and let it finish patching before you log in. Install outside Program Files to avoid permission problems.
Report exact details
If patching or logging in fails, open a support request with the exact error text, what you were doing and your system details.