If your HD streams are maximum of 20Mbps and you have four recordings then you will be reading 4*20 from the tuner (=80Mbps) and writing the same to either disk or Ethernet (Pi shares Ethernet processing with USB bandwidth). So that makes a total of 160Mbps. If you're playing back two streams too then that's reading 2*20Mbps from disk and writing 2*20Mbps to the network. So that's a further 80Mbps.
So the total you have is:
USB2: 480Mbps
Recording: 160Mbps
Playback: 80Mbps
So total USB bandwidth would be 160+80=240Mbps of the 480Mbps (excluding overheads of the protocol of say 10%l).
I'd say it could probably handle it (since we're measuring peaks rather than average bitrates), but you might get "bus resets" where something times out and then reconnects. Remember also that data isn't written direct to the disk, but is cached by the kernel and only written periodically, say every 30 seconds and this has an impact on your peaks.
I'm surprised your USB disk is so slow but that is probably a limitation in the Pi. I'd expect twice that speed.
For filesystems, I've used most of them over the years. There's practically no difference for a home user since the throughput of broadcast data is so low.
Assuming you are sticking with Linux, nowadays I'd recommend "btrfs" since it allows you to easily replace one disk with a larger disk in the future with one command, whereas most other filesystems need you to do relatively tedious sync of data and editing of config files and shutdowns. I don't know how well it performs on the Pi. Otherwise, ntfs-3g would allow you to take the drive and plug it in to your Windows box if that's something you might need to do.