I assume you're running BT4beta? This a very well known issue and searching the forums would definitely help. Please verify by yourself if the issue is caused by the Pearsoncomputing PPA - this is just speculation at this point as you don't give appropriate information about the BT4 version you're using. If we're talking about BT4beta and the Pearsoncomputing PPA, this command may help
Update the package index once again and see if the problem is gone.
# apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 8882eef08796ef73ad26ab98cb2f6c86f77b1ca9