According to the analysis, Xiaoming's time in the library is: 100-40=60 (minutes);

Xiaoming walked 4 kilometers from home to the library, which took him 40 minutes, that is, 2 3 hours.

The journey home from the library is still 4 kilometers, and the time it takes is 120- 100=20 minutes, which means 1 3 hours.

So the average speed is: 4 ÷ 2 3 = 6 (km/h);

The average speed of going home is 4÷ 1 3 = 12 (km/h).

So the answer is: 60,6, 12.