Good morning to you also @drauth, wherever you are!
I wanted to know if it is possible to disable Universal Analytics (GA3) and enable only GA4, with or without the Google site Kit
You can certainly choose to not allow Site Kit to insert a Universal Analytics snippet (UA, as opposed to GA3) if you wish. By doing so Site Kit will not insert this snippet on your site, and no data will be connected for that UA property. You can still choose to allow Site Kit to insert a Google Analytics 4 (GA4) snippet if you wish, the below is an example of this setup, all configurable via your Site Kit settings.

Note that at present the Site Kit dashboard will display data only on a UA snippet. Once the GA4 API allows for the same features Site Kit uses to display it’s data, and once more users migrate to GA4, the Site Kit dashboards will populate data based on the connected GA4 property. For that reason if you do decide to use Site Kit to only insert a GA4 snippet, and not use any UA snippet, the Site Kit dashboards will eventually show no UA data. For example if you switch off Universal Analytics today, and view the Site Kit dashboards 7 day report, it will display no data in one weeks time (as there will be no data for the previous 7 days).
Let me know if you have any further questions on the above, and I’d be happy to provide further assistance.