An excellent white paper explains about Business Portal Customization. It has information on both Microsoft Dynamics™ GP 9.0 with Business Portal 3.0 and Microsoft Dynamics™ GP 10.0 with Business Portal 4.0.
The purpose of this document is to clarify what type of customizations can and cannot be done with Business Portal in Microsoft Dynamics GP. Some customizations are possible by implementers while others are possible through ISVs or Microsoft Dynamics Advanced Product Support Services.
This document will describe the customization options and Advanced Product Support Services available as well as which types of customizations are not possible through any type of service.