What are "consecutive input pulses"?
The use of the word "consecutive" might be a bit confusing.

The condition means that the output goes active after four consecutive pulses, of which at least two are on x1. In other words, it doesn't simply need to see two pulses on x1, there have to be two x1 pulses in the last 4 pulses.

Thus the sequence:

X1 X2 X2 X2 X1 

contains 2 X1 pulses, but not within 4 consecutive pulses, whereas

X1 X2 X2 X1 contains 2 X1 pulses within 4 consecutive pulses.