What is the purpose of the concrete? Is it a finished surface or a
substrate for setting tile? Why do you need 3-4"?

I want a solid surface on the balcony to keep anything from dripping
through. I'm having lightweight concrete poured on the second floor as
a sound barrier so it seemed like the thing to use. The location is a
couple blocks back from the Pacific Ocean which means wind, salty air
and lots of rain, fog and moisture - maintenance is an important issue.
I don't want to be painting something every year.

The 3-4 inches is not my idea. The concrete supplier says that is
usually what they pour on outdoor applications. I found a building
nearby with concrete balconies and they look to be 3 inches thick, with
a cold joint every 6 or 8 feet and had cuts in the top closer than that
( I didn't measure).