Nothing wrong with using primaries, it only limits your resizing/additional partition options a little later on. If the disk is only going to be used in a 64 bit Win Vista/7 machine, you can partition GPT and make them all primaries. GPT's limit is 128.
To do what you were originally describing, make the first primary 100GB partition, then make a 400GB extended. Then, inside the extended, make your four logicals.
Behemoth - Plex server
Galactica - FreeNAS
Pegasus - R710 ESX host
fsociety - MBP 15
Rocinante - E5530 testing machine
Saratoga, Ticonderoga, BunkerHill - the Hydra desktop PC-thing.