here is a NAND made in DTL from a german wiki page
we use "U" for voltage and "V" for valve.
I understand most of the circuit. R1 keeps the first node at 5V until V1 or V2 conducts. Which happens when U1 or U2 go to ground. V5 and Rc work as an inverter.
I don't understand what V3 and V4 do. Are they supposed to decrease the BE voltage of V5 by two diode-drops? And why goes R2 to ground? Shouldn't it be in series with V4 to limit the base current? Well, R1 already limits it. Then ... does it work as a voltage divider? But if yes, why V3 and V4?