I replaced the modulator valve due to internal leakage. I made sure that the adjustment pin was the same measurement as the old one. On the test drive it shifts good into 1 and 2. On the 3 and 4 shift it seems to be slipping some. What kind of adjustment can I do to the modulator to fix this?

Run the adjuster in (clockwise) about 2/3 to one complete turn.

Another possible adjusment is "when" it shifts. If it is shifting late and you are letting up on the gas during the slushy shifts then you can adjust the vertical link from the throttle linkage to the proportioning valve at the back of the pump. Making the link shorter will make the shift earlier.

The concept of these two adjustments (and at least one more that I won't get into) is to make your third shift harder without making the first shift unbearable.