Hi,
1) This depends on how you have the scene/options setup. Take a look at this manual page:
http://render.otoy.com/manuals/Maya_2_0/?page_id=364
2) Be careful with triangle vs poly. Octane has a limit of around 19Million triangles, so about 10mil polygons. Geometry, render target and textures all take up VRAM. Also, your render target will increase in size if you have lots of passes enabled. 1 million triangles is roughly 130MB
A normal 1920x1280 render target takes about 50MB but with lots of passes enabled this might grow to 500MB or more.
A LDR (24 bit RGB) image will take up about 80MB - but octane has an out-of-core textures feature which means it can use textures from main memory, leaving more VRAM free for your geometry and render target. Or just plain allowing you to use a lot of very high res textures.
3) It will still be 4GB. Octane needs to copy the data to each GPU so it can render the scene. It can't reach over to other GPUs and share VRAM.