If you have a spare step ladder laying around set it up in the coop for awhile. Also add one or two more roosts but lower. Maybe the first roost is just a bit too high for their first experience, your birds may just not be that adventureous. The roost isn't too high for normal situations, but since your's do not use it, try adding another one lower. At least for now, once they start roosting they'll go higher.
Another theory that may be in play here, they may view the roost box itself(4 ft off the ground) as sufficient height. How large is your roost box? Is it large enough to lock them up for a couple days comfortably? It could be that spending all day 4 ft below the roost box and then going into the roost box(up 4 ft) they don't feel it necessary to perch. If this might be the case, try penning them up for a couple of days in the roost box. By penning them in the roost box, they'll stay on the roost box floor during the day and then the urge to roost will send them up that extra 2 ft to their perch. Make sense? This will probably only work if their is natural light inside the roost box or a light.
Keep me posted, we will figure it out! Poor Ladybird will get in the house yet!
-Kim