The size of the structuring element can be whatever is suitable for the task at hand. That's why it's user-specified. While strel() accepts this parameter as an integer-valued radius, bear in mind that the actual structuring element will always have odd diameter. In that sense, a radius of 4 shouldn't seem strange in that it's even. The array size is still odd.
st = strel('disk',4);
st.Neighborhood
