IGS does have a four shelf unit. Both the 3 and 4 shelf units are 72" high, which gives you less space between shelves on the 4-shelf unit. The advice I was given is that the 4-shelf would not give me enough space between shelves for standard violets. For minatures, the space would be enough.

So a 4 shelf unit is nothing more than a 3 shelf unit plus 1 extra shelf. Imagine that. Thanks for the information.