4 lbs is exactly what your baby should weight at this time. However we cannot estimate really how much ur baby will weight at birth. if he stays on track for the rest of the pegnancy, then he shloud weigh anywhere from 7 or 8 lbs
The average weight at 32 weeks is about 3.3 lbs, but remember, ultrasounds can be off by quite a bit when dealing with weight. =)