Losing boost in 4th gear?

If everything is working properly, the amount of boost will vary depending on the rpm. At higher rpms, you should see the most boost. First and 2nd go by so quickly that you shouldn't even bother looking. In 3rd, you are probably looking at the boost as the tach needle is swining from 5000 to 6000rpm. Are you doing 5000-6000rpm in 4th too? If so, shame on you for doing 120+mph on the street, but you may have a problem. If you wern't past 5000, then you probably would reach 6-7psi in 4th if you were to take it that fast.