Step-by-step explanation
The total area under the normal distribution curve is 1. z-scores are indicated in the horizontal axis below this curve. This means that the sum of areas under the curve at the left and at the right of a certain z-score must be equal to 1.
Then if the area at the right of the z-score that we are looking for is 0.04 the area at its left must be equal to 1-0.04=0.96. The area at the left of z is important because z-score tables usually show the areas at the left of several z-scores. Then the only thing that we have to do is look for the z-score associated with 0.96 in one of these tables. In your case the table that you should use is the one named "Normal Table -∞ to z". That table should look like this one:
As you can see the value 0.96 is associated with the row 1.7 and the column .05 which means that the z-score that meets that the area under the curve at its right is 0.04 is z=1.7+0.05=1.75.
Answer
Then the answer is 1.75