If you have a column of warband and the front one is in combat, then the next two are in the TZ of the enemy element.
Now, the moves allowed in a TZ include (bullet 3) "line up in close combat with an enemy front edge its front edge has contacted, or as an overlap". From the wording alone, it looks as though you can move into overlap in any circumstances, but comments made by Phil on the yahoo group imply that it can only move into overlap on an enemy front edge its front edge has contacted. This means that the 2nd and 3rd rank can't move in any of the directions allowed for spontaneously moving elements (because of TZ constraints), so they don't move.
As elements 1-3 in the column are not moving, a 4th rank element is permitted to move into overlap.
This conclusion is also stated in the DBMM Commentary document.