Let's suppose you have a population of 1000 people.
If the incidence rate of the disease is 0.9%, it means that 1000(0.009) = 9 people will have the disease
It means that 991 people will not have the disease.
Of the 9 people who have the disease, 4% of them will test negative, then 9(0.04) = 0.36 people will test negative, so the remaining 8.64 people will test positive.
Of the 991 people who don't have the disease, 991(0.02) = 19.82 people will test postive.
Then, the probability will be:

The probability is 0.30358 or 30.358%