The difference between the Sessions metric in UA and GA4
The session counts in your GA4 property may be lower than those in your UA property. This can be explained by the fact that GA4 does not create a new session when the campaign source changes mid-session, while UA does create a new session under that case.
If a session crosses a day boundary (e.g., if it starts at 11:55 pm and ends at 12:05 am), it is considered a single session, though it is counted once daily. Thus, a single session can be related to different traffic sources and multiple days. As a result, the session count will differ from the unique number of sessions.
GA4 properties use a statistical estimate of the number of sessions on your website or app by estimating the number of unique session IDs, while UA properties don't estimate the number of sessions.