FWIW, I checked my Quick Scans which normally scans in the neighborhood of 14,000 to 15,000 items.
Yesterday (June 11) it scanned only in the neighborhood of 9,000 to 10,000 and the same for the one Quick Scan that was run this AM.
This is about a 5,000 item drop as compared to 4,000 items for you - so I'd say comparable.
I am running a manual Quick Scan @ present and will let you know what I end up with. On a Manual quick Scan I came up with close to 14,000 total items.
One thing that may help explain it is to go to History > Scan Results > double click a Quick Scan that showed the full 9000 items, left click the + by Total Items Scanned and that will give you a break down of how many of each category were scanned. Now go to one of the Quick Scans witht he 5,000 items and do the saem thing and see if you can find which items are not being scanned and/or which categories have fewer items scanned.
Below is what mine looks like - you can see where the differeences lie:
I know the manual quick scans are not recorded in the History, but you can get the same info via Result Summary after the quick Scan shows completed.
Anyone got some reasons?