Hello;
maybe you use cpanel or plesk.
If so, unfortunally the 4images script and your FTP program cannot do anything. Folders' rights are in this case managed by cpanel or plesk.
You will so have to connect to your cpanel or plesk interface, delete data/media/cat_id and data/thumbnails/cat_id folders, and recreate them assiging them all permissions, where "cat_id" is the number assigned to your category.
If I am right, in your case, 4images script and your FTP client are detected as "apache user" and not allowed to change directories permissions.
Another way to solve such problem is to change your host... Sorry my answer did not give you any immediate solution.