The linesman nearest the ball/play would have greatest say, up to the referee to have final decision (He could allow play to run and check with linesmen after, as in Amrican football, but dunno if I like that so much). Four linesmen would mean that dodgey offsides, fouls etc where a linesman is usually looking to the far side of the pitch, should be eradicated as the other one was nearer, also the other linesman could watch the offside while the other was watching play nearer to him and each could see if anything happened "Blindside" to eachother. Anyway, refs and linesmen back eachother up to the hilt most of the time anyway so if one "Senior" linesman put a flag up the "newbie" would probably copy automatically anyway.