I did not do the 4D (in fact, I don't know what is it advantage) but I do think that structure ultra sound is essential as it can view the baby situation in a more detail way e.g.
- is the heart running and function OK
- Kidney
- whether have 10 fingers and toes
- view the spiral code
- view the brain........
I personnaly feel it worth it.
Can 4D do the same? If yes, you may just do the 4D