jq '.metadata.annotations += { "label4" : "two" }'

cat data/ann.json cat data/ann.json | jq '.metadata.annotations += { "label4" : "two" }' 
{
  "metadata": {
    "annotations": {
      "deployment.kubernetes.io/revision": "1",
      "machine.openshift.io/owned": "",
      "label1" : "stam",
      "label2" : "another",
      "label3" : "one"
    }
  }
}
  
    
{
  "metadata": {
    "annotations": {
      "deployment.kubernetes.io/revision": "1",
      "machine.openshift.io/owned": "",
      "label1": "stam",
      "label2": "another",
      "label3": "one",
      "label4": "two"
    }
  }
}