Is it possible to increase the amount of addressable ram in unity?

What system are you using?

On Windows (and I suspect Macs) a 32bit OS can only address 4gb of ram, however you never get all of that, as some is reserved by your gpu and other hardware. So if you have a 1GB gpu then you'll only get 3GB, maybe less. Then of course the amount of actual ram you have is even less since the OS and programs use it.

The only way around this is to update to 64 bit hardware and OS.

As for building issues, there have been a few questions and potential workarounds posted if not here then in the Unity forum, which would be worth searching for and reading.