BSD has to be loaded on a primary partition. 1-4 That's all, period.
Also, BSD uses the term slice for what you call partition. And within the slice, you create partitions for /, /home, etc. Screwy from Linux, but works great. You can have 4 different BSD's on 4 different primary partitions (called slices in BSD), then within each BSD slice, you create your partitions for /, /home, etc. So... you'll want to make a partition (BSD slice) big enough to hold all your partitions. yeah, gets confusing at first.
Before you dive off into BSD, read (scan) through the Handbook at FreeBSD.org. Its probably the best documentation you'll ever see. Partitions are covered very well.