SELECT * is considered by many to be a bad habit because unless you end up using every field from the table you are returning more than is needed and that cost resources.
If you want full name you do
SELECT fullname FROM cas_user_ext WHERE x_id = 4
Or to get multiple fields do
SELECT fullname, email FROM cas_user_ext WHERE x_id = 4
You may want to go through some SQL tutorials to learn how to do basic CRUD operations (Created, Read, Update, Delete)