From what I can determine so far the only place that has the BAC-4000 and 8000 tuned or made plug and play for the Sur-ron is this store.
Looks like $200 difference between the two. They have a contact link on their page and would best know if the 8000 fits. They reply back quick if you submit them a question. The stock Sur-Ron controller is huge so I see no reason why the 8000 would not fit. The 8000 is not so much bigger than the 4000, but best to just ask them.
The 4000 has a ridiculous amount of power, the 8000 even more reidculous. The 4000 can hit something like 32KW peak. The 4000 is probably all you would ever need but the 8000 may allow more acceleration from 0-5MPH because of more phase amps. You would need some major battery upgrade to probably ever get use out of the 8000, you would also have to sacrifice on capacity to get higher current rated cells.
I wouldn't overlook the need for more phase amps of the 8000. The Adaptto Max-E had software limited phase amps compared to a lyen 18fet that I used and I noticed immediately the loss of 0-10MPH acceleration with the max-e. You get used to it but my bike didn't have that dirt bike feel and power to it with the adaptto limited phase amps. Battery amps are good for top-speed, but low speed power and fun is all about phase amps. I can't say if you even need the extra phase amps of the 8000 because the Sur-Ron is a mid-drive and they may not be nearly as important as on a hub motored bike. I personally would probably buy the 8000 just to not be phase amp limited.
I assume you can buy the 4000 or 8000 elsewhere?? but it may not be plug and play with wire adapters and tuned for the Sur-Ron.
You can see in their description for the 4000 that they custom tune it for the Sur-Ron for things like regen, eco / sport mode, how much this matters I have no idea how easy it would be to buy a 4000 elsewhere and do it yourself. Can you even buy the 4000 at another store cheaper, I can't seem to find any online.
Good link on more information on the 4000 & 8000