If you know how D4 is defined, the answer becomes self-evident
Further down you’ll also find the definition of TX1 for the Xenon.
As long you don’t call Serial2.begin() D4 won’t be used as TX1 but the pin number would in both cases be 4.
You can write this obscure code and it would render perfectly valid results.
pinMode(D4, OUTPUT);
pinMode(D4, LOW);
digitalWrite(TX1, HIGH);
if (digitalRead(D4) == LOW)
Serial.println("This will never happen");