The Invalid Payload message occurs when the GP4 download was either interrupted, corrupted, or failed to complete.
In order to correct the issue, clear the browser cache and download the GP4 program again.
If the problem still persists, you may need to try to download using a different browser, such as Firefox or Google Chrome as the current browser may have something preventing the complete download (virus, malware, etc).

This article shows how to clear the cache on most popular browsers.

The Firefox web browser can be downloaded here: http://firefox.com
The Google Chrome web browser can be downloaded here: http://www.google.com/chrome