I get a prompt that says Lightroom 4 Catalog cannot be opened because another application has it opened. However I do not have another application using it. I tried uninstalling Lightroom and re-installing with no luck. Any ideas. Thanks
Go to the folder with the catalog file in it, then with LR closed, delete the catalogname.lock file, then restart LR.
When LR opens it creates this .lock file and when it is closed, normally, the file is deleted. This is to prevent two LR's from trying to open the catalog at the same time, but if LR aborts unexpectedly then the .lock file can remain even if another LR is not open. This is probably what happened in your situation.