Height of a 2-3-4 tree
Citing from Wikipedia (https://en.wikipedia.org/wiki/2%E2%80%933%E2%80%934_tree)

Every node (leaf or internal) is a 2-node, 3-node or a 4-node, and holds one, two, or three data elements, respectively.

Thus, since the squares in the bottom row do not hold data elements, they are not nodes of the 2-3-4 tree.
Then, using the definition of height from https://stackoverflow.com/questions/2603692/what-is-the-difference-between-tree-depth-and-height the height of your tree is 2.