You can add an existing matrix size similar to what you need, like the 3x1, then right click the inserted matrix and select linear.
This will give a simpler representation of that matrix, with at signs (@) signaling the start of rows and ampersands (&) signaling contents of a column/cell, which you can then try and add to by mimicking the style.
When I created a 3 row by 1 column matrix, this is what the linear format looks like:
■8(@@) -> ■8(@@@) So I can just add another @ symbol to create another row to make it a 4x1 matrix. I am not entirely sure what characters the black box and 8 represent, but if you leave them, moving back to professional mode seems to work.
One you have turned the 3x1 matrix into a 4x1 matrix, right click it again and select professional. This should change it back to the more visual style you desire.