The upgrade process adds several database tables; not sure if installing an older version of WCS would strip them out. There is the possibility of the downgrade corrupting the db. You may want to open a TAC case and inquire about it.
The 2 feasible options would be to either do a fresh install of the desired version and restore a pre 5.x db backup, or just start all over with a fresh 4.x.x.x. install
You could also try building a test machine with 5.x, make a backup from the production wcs server and restoring it to the test machine and see what happens when you try to downgrade to 4.x.x