Same problem for me too.
Cache cleared, V4 Mautic
Get to API Contacts or any other, trying to pull record for a contact it returns the same Error not found.
Trough command
GET with ID of the person.
But I think I figured out the problem.
It might be server settings related and depends how your server (web server, like apache or litespeed) is configured.
So what definitely does not work:
Clearing cache from documentation (rm- rf var/cache) does not work for sure
Running command under the user (NOT as root) in apache webserver will be successful, but sometimes, I do not know exactly when as it happened more than once but could not replicate the problem when I wanted, new prod folder under cache was created with other permissions (not a web server one and not root and not user). Seems this happens only when you run php clear command and maybe when that php is running by another handler and not under the user of the public_html.
So solution:
Definitely run clear cache from command
Check chown of the prod folder under cache which in my case was the issue
I think this happened when person installs Mautic by unzip the folder directly on the hosting account.