Excuse me for using a Slackware link, but this guy has a knack for explaining clearly
read only the part about partitioning.
One of the nice things about Slack's old fashioned installer is it makes you stop and think about what you're doing.
Yes, 4 primary partitions is the limit. 4 primary partitions could give you 3 OS and a swap, if you choose to go that route. There are good reasons for using a more complex partition scheme. For instance keeping /home on it's own partition can be handy.
I have 3 linux OS installed sharing a swap that is on an extended partition. Can't tell you whether this is optimal, but it does work with no noticable problems.
Edit: Adding to the bootloader can be as simple as this
note that if the other OS's are setup with LILO you are back to manually editing menu.lst
Will post back when yesterdays beer wears off. Things aren't coming easy at the moment.