Some people get confused about what a tier is and what a layer is. A tier is more of a physical separation in your system. A layer is more of a logical separation in your system. People in the
JEE world always refer to the "presentation tier", because it is the web module of a EAR file in the end of the day. It involves things like JSPs and
Servlets. The "business tier" corresponds to the EJB module of your EAR file, and involves things like EJBs and MDBs.
In this system, the only physical separations that exist are between client, server and .db file - hence 3 tiers. Logically, you can have 4 layers: presentation, business, persistence and .db file (I know the .db file is more of a physical separation, but the traditional 4-tier layered architecture style also refers to the database as a logical layer).
And finally, please take a look
here. I think it might be helpful!