A 2 3 4 implemented in C++14. Based on these slides on 2-3-4 trees. There is a great step-by-step animation of 2 3 4 tree is at: http://cs.sou.edu/~harveyd/classes/cs511/docs/234Tree/tree.htm
Code was compiled with g++ version 5.3 using -std=c++1z option.
Everything seems to work, including the iterator classes.
Further documentations is at Implementing a 2 3 4 Tree in C++14.