Without a proper plug chop - the lighter ones look to be more correct in colour so then it´s most probably over fuelling on 2&4 or could be an inlet / exhaust restriction... if there are physical links - I´m not familiar with the manifolding on the sigma, are there any common themes on inlet / exhaust to 1&3 vs 2&4?
Other things to think about... - Do they run off the same coil? Is the engine management system set up correctly are they all firing at the same crank angle? Is the fuel injection semi sequential and could this be a clue? I seem to recall ford have a different 4cyl firing order than 1,3,4,2 (1,2,4,3?)..?
Sorry I´m not being much help without reaching for Mr. Google but maybe some food for thought???
Edited for clarity
Edited by - Neil66 on 26 Feb 2013 21:24:16