Hello,
If the events s1, s2, s3, s4 are each independent from r then so is their intersection.
Therefore from the formula of independence : P(s1 and s2 and s3 and s4 | r) = P(s1|r)*P(s2|r)*P(s3|r)*P(s4|r)
Further, due to the same hypothesis the expression becomes:
P(s1 and s2 and s3 and s4 | r) = P(s1)*P(s2)*P(s3)*P(s4)