The Multi-Boot Configuration Handbook

Minimizing Primary Partition Consumption

The PC's partitioning system limits the user to only four primary partitions. Many OSs, and even some disk utilities such as IBM's Boot Manager, consume a primary partition. These two facts combine to make primary partitions a precious resource, one not to be squandered needlessly. As a general rule, if you can think of two ways to partition your system to achieve a desired result, you should use the method that consumes fewer primary partitions. Other concerns do sometimes intervene, however, so you might need to balance this issue against disk speed, obtaining consistent drive letters across OSs, or some other factor.

OSs That Require Primary Partitions

Many OSs must boot from a primary partition. ...