misfire that won't go away..

[Muddi Camel]

help...... please
I've got a low mileage foxeye that's not been used over the winter. Battery's been trickled all the time, so it starts on the button, but runs rough. It was all serviced & balanced at the end of the summer and ran beautifully. 
The plugs seem clean still, but pulling #4 lead off and there's little change in engine note - significantly less than the others. The leads are doing their thing, and I can't see how a dormant plug could die over the winter....
As there seems to be a spark getting to each plug, I suspect fuel.
I've just put a load of fresh fuel in...  no change.
Is there anything else I'm missing?
Thanks