This is a free, live, interactive tool to help you with App-V 4.* Client Errors.    It requires a web browser with Silverlight.

Enter error code and see causes, troubleshooting suggestions, and links to KBs.    It also contains a flow chart showing the client processing that leads to many of the most common errors.  This can be helpful to understand at what point the client was when the problem occurred. 

This tool does require that you have Silverlight, but you can install to run off-line too.  Just right click on it to install locally.

Click here for: A Failure To Launch tool.