Citroen c3 cutting out/stalling?

I have a 2003 citroen c3 1.4 petrol,over a period of 3 weeks my car stalls and cuts out when slowing down for islands /junctions etc.It does restart but is obviously quite dangerous.My local mechanic has come up with a fault code of P0204 so has replaced injector 4,and has replaced coils/plugs etc.The same fault code still comes up.The car also ticks over very lumpy and cuts out,it sounds and feels like there is a cylinder down.Thanks in anticipation for any answers.