What do the 1D filters represent when using imfilter?
If I'm not mistaken, a column vector will filter the image across its columns, treating each row independently of the others. Likewise, a row vector will filter across rows, treating all columns the same. 

edit: Regarding an example - consider the simple image [1,1,1;0,0,0;-1,-1,-1]. It's constant along its rows (i.e., all the columns are the same) and a gradient along its columns. Let's filter it with a differentiating filter kernel [1,-1] along rows or columns. If we do imfilter(I,[1,-1],'circ') we obtain the zero image: since the image was constant along rows, filtering each row with a differentiating kernel gives the zero image. Oh the other hand, for imfilter(I,[1,-1]','circ') we obtain [1,1,1;1,1,1;-2,-2,-2]: each column gets differentiated independently and since all columns are the same, the resulting image is constant along rows.

As for the 0.25: this is merely a scaling of the whole image. You might as well filter with 1 as a filter weight and divide the result by 4, the effect is the same.