I have been tracking through GTM the visits to my pages while reporting a userId that my platform is pushing everytime the page loads within the tag. To do so, I followed the regular procedure of adding the userId as a custom dimension in the datalayer in GTM and it worked beautifully.
Ever since transitioning to GA4, I can't seem to retrieve this dimension anymore, even though I can see it is collected in the debugger whenever I visit a page. In any report, this dimensions comes up as (not set). It works correctly for another dimension called companyId.
any idea where it might come from ? 
The screenshot comes from the debugger, in the payload of my pageview custom event.
The way of manage the users at GA4 if fully different and much more "automagic" Follow this guide in order to implement it https://www.analyticsmania.com/post/google-analytics-4-user-id i think is the more complete one And good luck ;)