I did mine at Baptist Hospital in Kowloon Tong, where I had all my prenatal work and delivery.
I think the regular scan is important for checking on the baby.
As for the 4D scan -- which I had around 30 weeks, the optiminal time for it -- I have mixed feelings.
It's kind of cool to see the baby moving in 3D in real time, and getting the DVD, etc. of the moving image. But I felt it was a little gimmicky and it didn't seem to be medically necessary. The doctor seemed awfully taken with his new machine and what it could do.
If you want a DVD "souvenir" then go for it. (Frankly, I never watched mine). But I don't know it you NEED it unless your doctor says there's a complication.