Is 4TB the largest drive you can use for this method?Correct. It's a 24 month deal with a 24 month warrantee.
I saw one thread where someone added an eSATA port above the existing eSATA port. My enclosure came with an eSATA port that is intended to be mounted in place of one of the card slots on the back of a PC. I took it off of the metal bracket and checked to see if it will fit in that spot in the Tivo, and it does fit perfectly. So, after my 24 month warrantee is up, I'll probably do that.