the 411 looks like it has more modern windows and doors, than the 421. With such little difference, and minor changes to the front what about contacting the 421 Author and seeing what they can do?
. . : :Simulating the UK's first dedicated high speed line for MSTS: : . .
Note the front end difference:
Cab Windows (Shape, size and position)
Multiple working pipes (recessed on 421)
Arched corridor doorway on connection
The 411s always looked grubbier and more basic, especially when telling the difference in rail blue and grey!!