The nurse just told you the estimated price. If you are not requesting vitamin or other special things, normally the charge will be 450 per time.
The 4D will really depends on the position of your baby, if it's possible to see the position like face, sex organ or you requested to see the foot/hand. If position is not suitable, it's also no meaning to let you see the 4D. 4D only see the outlooking of your baby.
I went there during my 33wk+, I am lucky that I can see 4/5 my baby face since if it's over 32 wks, it's quite difficult to see.