We can do a very rough estimate with some basic math and assumptions. Let's assume GCN 1-2, 2-3, 3-4, and 4-5 all have a roughly 7.5% efficiency advantage while 5 to Navi (RDNA) has 25% as listed above by Pema.
6 TF * 0.925 * 0.925 * 0.925 * 0.925 * 0.75 = 3.29 TF
So we're looking at roughly a 3.3 TF Navi GPU to reach similar performance to the XB1X, theoretically. 4x that is roughly 13.2 TF.
This is all extremely rudimentary and depending on other hardware changes made by MS/Sony, I'd wager anything between 12 and 14.5 TF would be the range for 4x.
That would be a "little" above the 10Tf ceiling a good portion is expecting, and 14.5Tf is above most expectations. But 12Tf is probably where the "majority" would expect both systems to hover about and would give the 4x gain of your calculation.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"
Azzanation: "PS5 wouldn't sold out at launch without scalpers."