What size is your oven and how many leftovers do you want? We buy a huge one for just two of us (and the dog)and eat all of it over christmas, first roast, then cold then in a turkey and chestnut pie then maybe curried and finally stewed. If you look on the sainted Delia's website, she has more info, alos a lot of turkey wrappings tell you how many people that size will feed but I don't think they allow for leftovers.
There are 4 of us at Xmas and we were told to get a 6lb turkey - seeing as they are in kg though, that would be about 4kg ish. We got ours from Tesco and although they have the weights on them, its done as small, medium or large. The small one's start at around 4kg, but they aren't exactly small small, if you know what I mean. Should be fine for 4 of you.