I don't have personal experience with this cultivar, but have seen them in person and I do not believe it will stay as small as stated.

While it may have a reduced rate of growth in comparison to normal buttonbush, I am still expecting Sugar Shack to be in the 6-7' range fairly quickly. They are a great native so if you can deal with a little pruning it would be a great addition to your garden!

Here's a link to an article on it in a Canadian journal, and the photos show it as growing more slowly than the comparison cultivar, 'Sputnik'. I've only been seeing references to 'Sugar Shack' since 2013, so I imagine that the jury is still out on its ultimate size in different parts of the US. I don't know anyone who has grown it. The plain species gets larger in southern areas, so this cultivar may get larger for you, IDK.

Everything keeps growing, when you read a shrub grows 4' tall that will always be a representative size for a certain period of time. Each kind will grow a lot less on some sites and a lot more on others, will live longer under better conditions, and so forth - none of them are tables and chairs, that remain the same size indefinitely, wherever they are placed.