Can I replace MS-DOS with 4DOS ? What is the difference between 4DOS and MS-DOS ?
This is the Frequently Given Answer to that question.
4DOS is not an operating system.
4DOS (just like COMMAND, CMD, 4NT, 4OS2, TCWIN16, TCWIN32, and TCOS2) is a command interpreter. It isn't an operating system. It is an application program, just like any other, that runs on top of an operating system. It presents a user interface that allows a user to enter command lines that it then interprets.
The name "4DOS" was originally a pun (literally: "for DOS"). It's a pun that many people regret, because over the past decade or so countless people have foolishly assumed that 4DOS is an operating system simply because it ends in the three letters 'D', 'O', 'S'.