Play By Web is probably the biggest multiplayer feature for SE4.
You can upload and download your order and turn files right from the web interface, or you can have the server email your turns to you and you can reply with your order file attached.
You can of course do it manually if you want, but PBW automates all the busywork of collecting and distributing game files.
Hotseat play is great fun if you can find a partner or organize an SE4 night with your friends.
Sequential turn with tactical combat vs your buddies can be quite entertaining. The ability to control all your pieces during combat head to head with another human is great
The demo should give you a good idea of the interface and some gameplay action. Definitely try it out, and stop by to visit the PBW site