4 PSI of boost will NOT give you an additional 60 bhp.
The formula for calculating an HP increase after boosting your engine is as follows
(PSI Boost) / 14.7=Percentage Increase "x"
x * Rated HP of your car
So for your stock VTi:
i) 4 / 14.7=0.272
ii) 0.272 * 128=34.82 additional HP
So your stock VTi would gain about 34.82 additional HP, give or take a few HP.