Without funding in the fourth year, do most PhD students work whilst writing up?

Well, that's what I'm planing to do. Some PhD students get lucky and their supervisor "finds" some money for them or employs them as a technician or postdoc. At my institute they want people to be finished on time so they don't start looking for money, honey.

I spent most of my summer marking exam paper Yawn... so I can have enough money to finish writing up, don't how long it will last though..

I think you have a choice between not working and writing up full time (and hence fairly quickly) or finding a job and writing up in your 'spare time'. The latter option obviously means that it will take you longer to complete but we've all got to eat so sometimes it's the only real option. Good luck.

I have funding for the fourth year..so will be doing some last experiments and writing up.

I am at similar situation, my scholarphip runs out in 3 months. My supervisor does not care and I will not get any additional money for sure, so I will try to write what I can and live from money I saved, and then go to work when I will have to.