I don't know that we have accurate statistics, but I can confirm that the vast majority of customers I speak to use Tomcat. As has been stated, Tomcat can make it simpler, the vast majority of wiki how to's, tips and tricks etc are actually with tomcat, so it is the quickest way to find information on with respect to BI4 deployments.
As per database licensing concerns, BI4 does ship with sybase by default, perhaps you want to consider that?