user44007: Is it always possible to find an hash value of a block, smaller than the current difficulty, just using 4 byte (Nonce) ? How can we be sure about it ? 

user12983: No, it is not enough.
There is extra-nonce field. The scriptSig of coinbase transaction. 

44007: <p>Is it always possible to find an hash value of a block, smaller than the current difficulty, just using 4 byte (Nonce) ? How can we be sure about it ? </p>
 12983: <p>No, it is not enough.</p>

<p>There is extra-nonce field. The scriptSig of coinbase transaction. </p>
