One of the biggest differences between UA and GA4 is how they measure data. GA4 uses event-based tracking, while UA works on a session-based approach. In Universal Analytics, sessions are effectively measured based on certain parameters, whereas GA4 estimates the number of sessions.
Some statistics should be relatively close to each other, while other metrics may show a larger difference. Google has also published an article comparing statistics in UA and GA4.
It's important to never interpret the numbers as absolute figures. You can never have 100% of all data because some people refuse or block tracking cookies. Look for trends to draw conclusions and use abnormal spikes to check if something is wrong with the website or tracking setup.