SegWit has a maximum block weight of 4MB.  This is not comparable to having a maximum block size of 4MB.  SegWit also cannot, in practice, reach four times the capacity that the legacy network had, even if all users were sending SegWit transactions.  

You could refer to the SegWit2x fork's alteration as increasing the base block size to 2MB.
To understand SegWit's effective capacity, try reading through this article.