BizTalk Server 2009 standard edition has a two CPU limit on it so you would think even if it was on a box with four CPU’s you’d only need two licenses. Not so you’d need four!! Read here a quote from the Microsoft BizTalk Server 2009 pricing and licensing FAQ:
The BizTalk Server 2009 STD engine can technically use only two processors on the server, even if it consists of more processors. For deployments where BizTalk Server 2009 is used, you need to buy the same number of processor licenses that Windows Server will utilize on that server. For example, if you have BizTalk Server 2009 STD installed on a four-processor server and the Windows operating system is utilizing all four processors, because the server’s performance is benefiting from all four processors running the operating system, you need to purchase four licenses of BizTalk Server 2009 STD.