IEEE 4 bus system - This is the 4 bus example from J. J. Grainger and W. D. Stevenson, Power system analysis. McGraw-Hill, 1994. 337-338. Its data origin is PYPOWER.
IEEE 6 bus system - data origin is PYPOWER. It represents the 6 bus example from 104, 112, 119, 123-124, 549 from A. J. Wood and B. F. Wollenberg, Power generation, operation, and control. John Wiley & Sons, 2012..