This question is made for me, I've been obsessed with the width of the blummin' things for years!
How narrow do you need something to be?
Narrower with carrycot and forward facing seat (or a pramette type thing) - there's a fair few once you start looking. The britax b motion is 56cm, cybex callisto is 53cm and the carrycot is gorgeous, the hauck Malibu is 54cm but the one I had was rubbish, maclaren xlr takes a cot but not carseat, ditto the babyzen 2014 yoyo which will offer a baby kit to make it suitable from birth. I could go on but stopping here I suspect you want parent facing beyond carrycot?
Reversible seat/book style fold and you're down to the bee, the easywalker mini at 55cm, the oyster at 56cm, the stokke scoot 54/5cm (i think?) and the silver cross wayfarer at 53cm.
If you look at an umbrella style fold (so the open pushchair is average width - 56-60cm - but the folded pushchair is a rectangular not box shape) then you have the new maxi cosi loola, also the Jane solo reverse and cross reverse, I think the mamas and papas switch easydrive might fall into this category but not totally sure. There will be a few more like this that just aren't springing immediately to mind, I think there's a babystyle one. The petite star bubble is good and narrow but no carrycot or carseat option.
The oyster gem folds small but 3 wheels where you wanted 4.
Any of that appeal or suitable? Originally I was bothered about folded width mainly, for the car, now I walk everywhere open width is more important. My dream would be a robust 4 wheeler with biggish wheels (possibly air tyres) decent suspension and a reversible seat that was about 52cm wide. The easywalker mini is the closest but once I realise that I realise I have a mental budget of under £350 and ideally much less than that :-/