Maximum Primary Partitions using four drives

I keep reading that the maximum amount of primary partitions on a drive is 4 and that's my understanding also.

Now suppose a computer had four physical hard drives installed on the two IDE channels....
that would be two masters and two slaves.

Suppose that each drive was partitioned with FDISK to have four primary partitions.

Does that mean you could boot from any of the 16 primary partitions by making a partition "Active" with FDISK? (Assuming an OS is properly installed)

Or, would you just be able to make one of the four primary partitions bootable or "Active" on the primary master drive using FDISK?

Another question
In a computer with just SCSI drives installed, how many primary partitions are possible.

You should end up with 16 primary partitions all of which would be "Eligible" or Capable to be bootable. Gots to have an OS or boot type files on the partitions ...