“For example one product was 0 at snapshot, PO was 8, but there is 4 on the shelf due to sales. So someone that is not involved in purchasing then enters count as 4.” In this case, you should count zero, because 4 was from PO after snapshot.
If the snapshot is 8, sold 3 after snapshot. You should count 8. Or count 5, and add 3 (sold) to another manual batch.
You should count items right after the snapshot created. If you cannot count the whole store at one night, you should do selective/partial count. Or re-create snapshot at the time point of actual counting.