docs: String task4(AwsSigner4Request sr, AuthConfig credentials) {
          StringBuilder sb = new StringBuilder("AWS4-HMAC-SHA256 Credential=")
                  .append(credentials.getUsername() ).append( '/' ).append( sr.getScope() )
                  .append(", SignedHeaders=").append(sr.getSignedHeaders())
                  .append(", Signature=" );
          hexEncode(sb, task3(sr, credentials));
          return sb.toString();
      }