Definitely use the latest version zizo.
The question is more complicated that 4000 users. How many will be on a the same time? ie concurrent users. See for example https://moodle.org/mod/forum/discuss.php?d=312130
Bluehost: an older post: https://moodle.org/mod/forum/discuss.php?d=197552 nd https://moodle.org/mod/forum/discuss.php?d=273815 (at random)
Check the docs: https://docs.moodle.org/28/en/Performance_recommendations etc
Why do you expect problems?
Good luck
-Derek