Mix together the four cheeses until totally combined and set aside.
Cut the baguette in half lengthways and scoop out some of the bread from the inside of the bottom half. Fill this to the brim with the grated cheese mixture and spread the uncut side with mayonnaise then place back together. Cut into manageable sections and place on a lined baking tray then brush with the butter and top with some baking paper and squash with a heavy baking tray. Bake for roughly 15 minutes or until the top is browned and the cheese is oozing out of the sides.