You imply you don't yet have
Sky, but I'm not quite sure whether the Dish and LNB are already there and you are just asking about Receivers, or whether you are starting entirely from scratch.
Either way, the only way to achieve what you want is to ensure that the LNB has four separate outputs (A Quad LNB) as each Tuner requires its own feed. That's two feeds for a
Sky+ box and one each for normal boxes.
This is because the boxes send switching signals back to the LNB and you therefore can't use a single LNB as there will be conflicts.
Also, can you please narrow down your location for us, as this potentially has a bearing on what size Dish you'll need for best results.