If they do that, and add two franchises, it's $4b once. That's just over $133m for each of the thirty ownership groups. Again, once.
How much is the difference in annual revenues when split over two additional franchises?
I think there's too much we don't know in order to accurately answer that question. The current deal has an AAV of $2.6B a season ($24B over 9 years). Divided 30 ways that is about $86.66M per season. Divided 32 ways it's $81.25M per season, a difference of almost $5.5 million for each existing team per season. So that basic math indicates it would be well worth expanding based strictly on the value of the current contract. Obviously they will be thinking farther into the future though.
And there are other little tidbits to consider. For starters, I'm basing the above off the AAV of the deal, but in reality it almost certainly pays less in the early years, and more in the latter years. So that means the current owners would give up more with each passing season. But again, maybe the payout escalates if there is expansion (with the thinking being the product is more valuable with the two additional fan bases from the new markets and more choice of games to broadcast)? And obviously that $133M up-front could go a long way if invested somehow (the old "a dollar today is worth more than a dollar tomorrow" principle). Finally, the TV contract I mentioned is US-only for national broadcasts....it doesn't include any overseas TV deals they might already have, plus NBA TV, League Pass revenues, league-wide licensing and sponsorships....all of those will be smaller pieces post-expansion.