Variation #1: why stop at 4? why not 2-3-4-5 trees? or M-way trees?

• allow nodes to hold up to M-1 items, and at least M/2
• if each node is a disk-page, then we have a B-tree (databases)
• for B-trees, depending on Item size, M > 100/200/400

• use standard BST nodes, augmented with one extra piece of data
• implement similar strategy as 2-3-4 trees → red-black trees.