Actually, what I want to know is, whether that whole of 4 MB required to validate and build a new block on top of that or just 1 MB was enough for that purpose?
To fully validate a block, you would need all of the block data, which can be up to 4 MB. That data is required to validate all of the signatures. To validate that the block header is correct (but not check signatures), you only need the non-witness data, which can be up to 1 MB. To verify only the proof of work, you only need the 80 byte block header. To build a new block, you only need the 80 byte block header, but basically no validation will be done.
