voltage - Why is D2 switched off and T4 not in saturation mode in 7408 TTL Gate as shown below?

In the circuit, when both the inputs are high at 5V, than why is D2 switched off? Will it not draw voltage from Vcc?

Also, when one of the input is low, why is T4 not running in saturation mode? Will it not get high base current from Vcc?