Cost depends on where you are located. A 4d ultrasound is a 3d ultrasound in motion. It a more accurate picture of the baby just in motion like a movie that they place on a DVD or jump drive so you can put it on your computer. they recommend you do it between 32-36 weeks that way the baby is big enough to see everything but yet small enough not to have their legs and arms squashed up by their face!
In my area for the simple plain pck was 50+ and they like to do it after 20 weeks, guess u could say it just a more in depeth(sp) look at your baby you can tell more details and such