Gotcha, that wasn't clear to me from your earlier post, but now I see what you were saying. With a project as large (and expensive as yours) its a lot easier to justify the cost of a top quality BMS as it wouldn't add much to the overall cost of your system. That's a segment of the market that doesn't get a ton of discussion or attention here.
It might be useful to think of a BMS as a system or design concept instead of a device, it can be a device, and as used/discussed on this forum, it usually is a device, but it can also be a decentralized system.
As to your questions:
1. How you hook the cable up depends on the BMS, Some BMS' are 'inline' (i.e. in the main current path), others (like chargery or SBMS0) are not and control dis/charging indirectly using relays or remote control of devices. I would assume with a system powerful enough to need 4/0 cable at 48v, you would not be using an inline FET based BMS.
2. The BMS does much more than just balance the cells, but that is one function
3. If by size you mean current rating, this is determined by the maximum current that will flow through the BMS at any one time. If the BMS is not in the current path, current rating isn't relevant (apart from rating of external components like relays/shunts/etc)
4. I know this is an unsatisfying answer, but the options you need will depend on your situation, needs, and goals. I highly reccomend you read the two bottom links in my signature, (nordkyn design and marinehowto) there is a lot of good info in those articles about how to protect and take care of lifepo4 batteries, and how to design a system around them, with some good discussion on what a BMS is and should do.