How to efficiently hash (SHA 256) in golang data where only the last few bytes changes

Assuming you had 80 bytes of data and only the last 4 bytes was constantly changing, how would you efficiently hash the total 80 bytes using Go. In essence, the first 76 bytes are the same, while the last 4 bytes keeps changing. Ideally, you want to keep a copy of the hash digest for the first 76 bytes and just keep changing the last 4.