jq --slurpfile users 1st.json '$users[0].users[0] as $users
     | .info.users |= map(if (. | in($users)) then . = $users[.] else . end)' 2nd.json

{
  "info": {
    "users": [
      "user1",
      "user2",
      "key3",
      "key4"
    ]
  }
}