<< If it a single 2TB drive is installed, I'll purchase an additional single 2TB drive which will increase the storage capacity to a total of 4TB. >>
NO, you won't. There is no 'open' slot.
The Boot drive in Mac Studio is an Apple Proprietary design. The modules involved are not standard drives. Generally speaking, if you want a larger boot drive, you will have to discard or sell your current modules, and install all new Apple modules. If any, only the very lowest model uses only one module.
Anything else you try to install in the proprietary drive slots will cause your computer to NEVER boot again.
Most of the firmware is written to a private part of the boot drive, so you WILL need another computer running apple confugurtor as well.
>> Please reed deeply the links that muguy posted, They contain all this bad news in detail.
That is why my previous reply listed other option s available to you internally.
Do you really need all this to be on the very fast and very expensive Boot drive?