Maybe those 4 are the total of the UK demo fleet to be divided by 160ish UK Fiat dealers. That's 1 car for 40 dealers so if FIat keep them moving around on the roads maybe there will look like there's more.
UK FIat dealer have been told they are getting 2 demos each in December .Some believe and Some realise now they have prob been misled again and are making own arrangements with left hander 500s to show people as Fiat can't supply anything else and prob wont be able to til next year.
If that turns out to be true it sounds like a right cockup.
Maybe those 4 500s were out on the razz or looking for some Minis to pester.
" kick em in the nuts and nick their lunch money" or something like that Mr Ford on Fifth Gear said. lol
Good spotting them tho mateu , most I have seen is 2 together at stanford hall show I think.