I haven't done mine at NUH specifically, but from what my mum friends have shared, the 4D scan availability really depends on which hospital and which package you're on lah. Since you're already 32 weeks, I'd just call their Obstetrics counter directly to ask if they offer it and whether you can still book one now, because some places only do it within a certain window. Your sister's experience at SGH doesn't necessarily mean NUH offers the same service or timing, so best to check with your own hospital directly rather than assume.
sng mnh