Answer
$0.3$
Work Step by Step
According to the first Rule for a Discrete Probability Distribution (see page 323):
$∑P(x)=1$
$P(3)+P(4)+P(5)+P(6)=1$
$0.4+P(4)+0.1+0.2=1$
$P(4)=1-0.4-0.1-0.2$
$P(4)=0.3$
Notice that: $0\leq P(4)\leq1$ (second Rule)
