I know that large creatures can choose to be affected by terrain, unless all of their spaces occupy that terrain.

Does that also count for obstacles? That is, can one square of rubble block a 4 square monster?

Yes, obstacles like rubble and water block large creatures' movement regardless of the tile size of the former.

^ What he said.

You are thinking of the Pit obstacle, which states large monsters are only considered to be in a pit if all of the squares they occupy are covered by pits.

Thanks! This seemed correct to us last night, so that's the way we played it. Nice to be vindicated