I'de say to delete your System_ReturnToBlockland.zip from your add-ons folder, Then download V4 from the website.
try running blockland as administrator
Tried that already, no luck :c
So you downloaded RTB4 from the website and installed it correctly, and it tries to update to RTB4? Sounds like you did something wrong.
Okay, now it's not showing up at all.
Press Shift+Tab and tell me if it does anything.