The probability of any value 1-6 facing up on a 6 sided die is 1/6 or ~17%.
P(<4) means "What is the probability that the die shows a number less than 4?"
In other words, what are the chances that the die shows a 1, 2, or 3
Since each side has a probability of 1/6 of showing, we can simply sum these 3 probabilities.
1/6 + 1/6 + 1/6 = 3/6 = 1/2 = 50%
So P(<4) = 50%