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As a practical matter, what are the advantages and disadvantages, of having all four, on my computer?

They are part of the Windows operating system, I recommend that you leave them alone. Deleting the ones you think you won't need won't improve the performance of your computer or free up vast amounts of space on your hard drive, if that's where you are going with this.

Powershell vs. Command Prompt
https://www.varonis.com/blog/powershell-vs-cmd/