This is a bit mixed up. The blocksize limit was replaced with a blockweight limit of 4,000,000 weight units. The theoretical blocksize maximum with segwit is around 3.7 MB with extremely convoluted transactions (we’ve seen a bunch of them on testnet before segwit was activated). Early estimates were expecting blocksizes of 1.7-2.1 MB at full segwit adoption. I’m not sure where you’re getting the 1.5 MB or 4 MB estimates from.