I've been looking around for PhDs for a while and one or two that I've come across describe themselves as "Teaching Assistanships" and most are 4 year courses. What would be expected from me on a teaching assistantship and how does it differ from a normal PhD?

Teaching assistanships generally are PhD studentships with limited hours of duties (mostly teaching) per academic year. Every institute have different regulations as far as I'm aware. Some does not allow you to take up any extra teaching work as the assistanship is paid on top of your studentship.