2
im not sure the exact syntax that will work, but what you are looking to do is called a subquery. Basically using the results from one query to provid some information to another and running them with one command.
I've tried contructing this for you but im not as smart as I thought, as I got syntax errors myself. But if you search for subqueries via google that might help.
my attempt which doesnt work right..
UPDATE xoops_users SET rank = 5 WHERE ( SELECT *
FROM xoops_users
INNER JOIN xoops_groups_users_link ON xoops_users.uid = xoops_groups_users_link.uid
WHERE groupid = 4 )