I can't help with FreeNas specifically, but if there's any advice I can give while this is still fresh in my mind, feel free to ask here or pm me and I'll do my best.
One of your hard drives will have the operating system on, that will be a waste of one of your 4TB drives. You might want to install an SSD disk in for the OS. I have one I haven't got around to installing yet.
I currently run Windows Home Server 2011 which is fine but I'm tempted to install something else at some point.
I use iHomeServer as an itunes server for the Apple TV and Cloudberry to backup to a NAS and Amazon S3.
Mine works great for streaming to Sonos, Apple TV, and my HTPC. I've not tried overloading it with multiple HD movies though. My main problem at the moment is that it keeps going to sleep even though it is set to stay on. It didn't used to so not sure what has changed.
Assuming you are looking at the gen 8 hp (if not, just ignore me as this probably doesn't count)... I think this depends on configuration. If you are using the 4 drives individually, I *think* it will want to boot off the lowest numbered sata drive it encounters, which will be one of the drive bays. To use an SSD you would have to use the sata connection for the optical drive, which is #5. If you are using the built in raid then it's not a problem - you can boot off any.
My box didn't come with a dvd drive, so I put the ssd in the empty space. Had to get a sata data cable and a floppy drive->sata power converter. If you want to have the dvd drive AND an SSD you'll need to sacrifice one of your hard drive bays or get some sort of expansion card.
It's worth bearing in mind that you can also boot off the internal usb or microsd (regardless of how you configure your hard drives). In the short term this means less messing about than fitting an ssd, but I don't know how advisable it is in the long term. Maybe someone who knows better than me could say if this is a good idea?