Re: 4-lug brake conversion
I assume you are asking if there are any kits to convert to 4-lug rear disc on the rear?
If so, yes. There are plenty. I believe there is a way you can do it with Thunderbird TurboCoupe rear brakes. If I remember correctly, it may require swapping the entire rear-end. There are also aftermarket kits out there, but they are expensive.
I would just go to 5-lug if I were you. It's much easier to find wheels and brakes with 5-lug.