It also provides a significant bump (maybe 75-80%
with that is like the block have size of 4mb
Ok, to clear this discrepancy up:
1.8MB is how big the overall segwit blocks could get, but that's when you run today's transaction burden through it.
4MB really means 1MB for transaction data, and 3MB for the signatures for those same transactions.
So, that means today's transactions use tx:sig ratio of 1:0.8 (5:4, in whole numbers). The segwit block structure encourages that to change to 1:3. In order to get the average ratio of transactions to signatures closer to 1:3, a much bigger average bytesize for the signature part is needed. But there's no point in doing that for the sake of it, only if it jams more transactions in per block.
Only Lightning transactions use transactions that are signature-heavy enough to make use of the 2.2 MB that 5:4 cannot make use of. So you can think of that additional capacity as almost like "Lightning only", or whatever other sig-heavy 2nd layer system can use it. And the ratio will likely have to change to reflect whatever works best in practice, but let's get this change behind us first, lol.
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