4 x 8 Is a really nice size. What are you planning to do with the shed? and how many piggies will live in there. My guinea pigs live in a 6x4 shed. I had it insulated and as I had 3 pairs, my dad built 3 6x2 hutches into the shed for them. And as he's an electrician I got him to run lighting and electricity to it as well, so they also have a small bar heater to keep the edge off if it's really cold at night. It's almost like keeping them in a cooler room, during the day they can have the door open if they can't go out and then if it's really rotten just have their light on and the window open for fresh air.
The main advantage to being out in a shed is that they are far better sheltered in the winter, but they can take advantage of go-outside-able weather for earlier and later in the year than indoor only pigs.