Herd had 18 months remaining of his current deal, but has earned new terms after impressing manager Alex McLeish, particularly since switching from right-back to central midfield.
The Australia Under-20 international told www.avfc.co.uk: "I am really happy to have signed a new contract and I'm pleased that I'm going to be here for the next four years.
"When we came back for pre-season the gaffer said he would give anyone a chance who deserves it and hopefully I can repay him and keep improving.
"Coming through the ranks the fans have been great for me, both home and away - and I really appreciate it.
"Growing up here, coming through the youth team, the reserves and then eventually the first team, they see that and they are happy about that to see youth players coming through."