Wanna do it with a retired pc? I recommend ps3 usb tv tuners. aka "Play TV" plentiful on second hand sights. Each has 2 tuners in it. Do you even need 2 of these devices to get 4? Ever had a 3rd thing to record or watch simultaneously? Might as well, they are very cheap.
xbmc with the myth plugin on the frontend is great if you own the thing that drives your tv screen. If you use appletv it seems you don't own it and can't run xbmc or mythtv frontend on it. But you can pay apple a sub and then also pay again for anything you might want to watch out of their stunningly limited selection. That's always apple's solution: Pay apple more to get something that isn't what you wanted...
xbmc with the myth plugin on the frontend is great if you own the thing that drives your tv screen. If you use appletv it seems you don't own it and can't run xbmc or mythtv frontend on it. But you can pay apple a sub and then also pay again for anything you might want to watch out of their stunningly limited selection. That's always apple's solution: Pay apple more to get something that isn't what you wanted...