Most toy haulers are geared towards ATV's, motorcycles and golf carts. Even a smart car would be too big for most.
The problem you run into is a snowball effect. You don't just bolt another axle on. You have may have to upgrade the beams that run the length of the trailer, that adds weight and cost. 4 axles is undesirable because when backing, the front and back tires will be drug sideways across the pavement (even 3 axle trailers experience this though to a lesser degree). You could go with 2 or 3 dually axles but then you need to push the beams closer together and that requires extra bracing to keep the frame from twisting. Things quicly sprial out of control.
With 3k bs of variable weight at the very back it creates weight balancing issues. You are probably looking at at least a 15k trailer to start with and a 3000lb pin weight (maybe bigger). The problem is if you take the car out and try to tow, you just lost 3k behind the axle so depending on the wheel spacing, you probably just added 2k lb to the pin weight, now you up around 5k pin weight and beyond most 1 ton trucks, so you are looking at commercial trucks.
As others mentioned, if money is no object, you can get custom made stuff but it will be very expensive. I doubt they will be making production units that will hold a mid size sedan any time soon.
