Driving lamps with the fewest number of NPN switch transistors
If you control the system with a microcontroller, you can avoid Q3 and Q4 by having one control line per digit. If the UC doesn't have enough output you can use a shift register.

You could use Mosfet / BJT arrays like this one to have fewer components but might be more expensive.



Alternatively, you can multiplex the lamps from the high side as follow:



You can multiplex at a frequency that is not visible to the eye (few kHz), although the overall brightness will be lower, you can probably drive the nixie harder as well.

Select proper Mosfet and use proper protection, especially for the high side. You can also use BJT.




Example for high side drive using BJT: (using random components)



simulate this circuit &ndash; Schematic created using CircuitLab