Can users without permission to see a topic see it if I @username mention them?

If I use @(username) does it add this user to the topic even if the topic is restricted to the tl4 level and the mentioned users are tl3?

If so, how can I delete them from this topic?

If you are referring to a restricted category, the answer is no, the restriction remains at the category level.

If you are referring to a message/private message, then a mere mention will still not add them, you must invite them for them to be added.