I actually tried this, back in the early summer, in my SE.
If you slide the front seats as far forward as they will go, and then push the seat backs as far forward as they will go, you have 1.8 metres (six feet) from the seat backs to the inside of the tailgate. I am five feet seven (1.7 metres) and I could lie stretched out in the space quite comfortably.
I have seen a video where someone put a double air-bed in a Trophy, but I think that's too much. There isn't the sideways room for a double, and if two people wanted to sleep like that it would be extremely cosy and there would be no space for anything else. You'd have to stow any luggage in the footwells somehow, or leave it outside the car.
Bjorn didn't push the seats forward and ended up scrunched up, even though he's not particularly tall. He did say something about doing that, but that you'd need to put something in the footwell to support the head of the mattress. He's right about that but I think a small flight bag would do it. You'd probably have one of these anyway if you were overnighting in the car.
I have both a single airbed and a lilo that is, I think, 2 feet 6 in (3 in?) wide and a smidgeon over six feet long. When the weather gets better I intend to try out both of these and see how they work. With the SE, what I'm not sure about is whether to put a thin pillow or cushion under the bottom of the lilo to bring it up to the height of the folded rear seat backs, or whether it will work better without in terms of the height - the higher the foot end of the lilo the less vertical space there is and the more the length is likely to be constrained.
I used to eye this up in my Golf, but I never experimented as I didn't think the length of the load bed was sufficient. Although the overall length of the MG4 is similar to the Golf the load bed is longer, as I could see when I put my bike in. Several inches more to play with. I think it's a runner. Not that I'd be planning on sleeping in the car, but carrying the lilo and a sleeping bag makes a lot of sense if you're unsure about getting accommodation on the road.
Now, does anyone know if it's possible to keep the aircon/heating on in the car all night without someone sitting in the driver's seat, or will it switch itself off? (The doors would have to be unlocked I think, or maybe using the central locking from inside would do the trick?)
) as you're 5' 2.5" or less you might fit in ... or lie down diagonally. 
