Look at the attached photo, it's reverse logic of what we were talking but the same truths hold...
As you see in a 4x2 matrix you check the keys like this...
All rows input
Both columns logic 0
Toggle column one to logic 1
Scan Row 1-4 to see if any of them are pressed
Toggle column one back to logic 0
Toggle column two to logic 1
Scan Row 1-4 to see if any of them are pressed
Loop
Thus you need the columns tied to I/O lines so that can change their state as you progress across them while scanning...
But in a 4x1 matrix you never need to toggle the column to scan all the keys, just tie column one to logic 1 permanently, no need to use an I/O line to do this as there is no progression across the columns as there is only one...