clarify block validity

EIPS/eip-4788.md

@@ -44,7 +44,9 @@ To bound the amount of storage this construction consumes, a ring buffer is used
 Beginning at the execution timestamp `FORK_TIMESTAMP`, execution clients **MUST** extend the header schema with an additional field: the `parent_beacon_block_root`.
 This root consumes 32 bytes and is exactly the [hash tree root](https://github.com/ethereum/consensus-specs/blob/fa09d896484bbe240334fa21ffaa454bafe5842e/ssz/simple-serialize.md#merkleization) of the parent beacon block for the given execution block.
 
-Validity is guaranteed from the consensus layer, much like how withdrawals are handled.
+Validity of the parent beacon block root is guaranteed from the consensus layer, much like how withdrawals are handled.
+
+When verifying a block, execution clients should ensure the root value in the block header matches the one provided by the consensus client.
 
 ### EVM changes