Wash off the slurry from the 4k side and from the station prior to moving to the 8k side and that should handle it. You definitely don't want a 4k slurry on an 8k stone. The converse is not true. It's OK to have an 8k slurry on the 4k side. Same is true with the blue and yellow belgian stones. OK to use the cotigura on the blue side but I wouldn't use a blue stone to get a slurry on a coticule.