Hello,
I believe the CSS needs updated for the Joomla blog layout. When you choose different number of columns, the BS4 template makes the last (1) or (2) grid blocks responsive instead of maintaining the column width at the bottom.
On another site, T4 BS5 made one of the blocks full width in the middle of the page. When you set column to 3,4 - whatever - it needs to respect setting on menu item.
I have also included a screenshot of the Cassiopiea template - which works fine as expected.
You can test this on your local installation. It happens on multiple project sites we have.
Regular Joomla Template (as intended)
T4 Blank BS5