What is the difference between 2 tier,3 tier and 4 tier architecture in MicroStrategy? | MicroStrategy

In 2 tier architecture, the MicroStrategy Desktop itself queries aganist the Data warehoue and the Metadata with out the Intermediate tier of the Intelligence server.
The 3 Tier architecture comprises a Intelligence server between MicroStrategy Desktop and the data Warehouse and the Metadata.
The 4 tier architecture is same as 3 tier except it has a additional component of MicroStratey Web.