Of course a 4 track everywhere system is better.

The question is do you want a 4 track system with half as many stops or a 2 track system with twice as much length?

It isn't quite linear scaling in cost, but you still need to physically make all the stations and platforms bigger, lay twice as much track, have twice as many signals and so on. You nominally get half as many stops and distance in your budget.

Generally the observation is that a dense network of 2 track systems is best. A good example of this is basically London. Nothing is quad tracked, but if a segment goes down you can go around it. They just built an ultra-modern cross London train (Crossrail/Elizabeth Line) it is 2 tracks.

For the WMATA system the step to add that is the "separated blue line" proposal. The system would benefit from a 2nd east-west main line though the city. Ideally touching all the lines. This allows transfers to get around a broken train or other issue in the core. It doesn't help you in the tails of the system, but they have less riders anyway.

Interlining is something that the WMATA system does a lot of that is unusual which increases the dependencies between them. In a separate blue line system Green basically has 2 southern branches (yellow) and 1 northern. Orange has two east and west branches (silver). Blue is on its own. Red is on its own.