Two wheels is fine for wheeling a folded bike. I have converted both my mezzos to four wheels and find I still tend to tilt the bike on to the rear wheels and move it holding on to the seat. A touring birdie out performs any bike I know of in this respect and that has two wheels not four.
I haven't had much experience with bromptons withour racks. However I did remove the rear rack and rear mudguard on one(see dahon andros stem on brompton).
http://www.bikeforums.net/folding-bi...er-advice.html
And
http://www.bikeforums.net/folding-bi...mudguards.html

If I have another Brompton it will be without the tiny, and heavy rear rack.

I tend to add rollerblade wheels. However the front ones on a brommie need consideration to avoid heel strike.
So.....

If you only use the outside bearing and no axle then a rollerblade wheel is as flush as an easy wheel afaik. You will need to superglue the bearing to the rollerblade wheel to secure it in place.
Would a rollerblade wheel say 70mm and the expander work well?
I looked like my brompton could be made to wheel ok on just two wheels with some mods. What about some sort of pull along handle like the Birdie front rack on a brompton?
Any one see something similar done?
I tried the front wheel only on my brommie but didnt look very hard for a solution before selling it. I got it very cheaply from a bango playing barber who used to race tandems.............
I offered to pay him 3 times what he wanted for it! But he stuck to his price!