Bike stored for 8 years, will run but only on 3 cyclinders.

[His Dudeness]

Have you tried swapping the plug caps?
What does the plug look like when you get it out?
I'd it dry or wet?

yep try that. the caps twist off the leads. give the ends of the leads a trim. try starting it. if it's still the same after that, try dribbling a small bit of petrol into cylinder 4 and see if it goes. if it does then it's likely a carb issue like a blocked idle circuit jet