I. of course, it isn’t anywhere near as simple as this in real life
II. the diagram is commonly called ‘the stack’, and the people in the computer industry love to talk about it
III. at the base are components that are assembled into finished hardware products; operating systems, middleware, and software applications sit above the hardware; and it’s all topped off by a whole range of services
IV. the stack shows most of the major pieces in a typical computing environment

The given passage talks about the diagram called Stack and how it is organised.

Statement II introduces Stack.

Statement IV gives a basic overview of Stack.

Statement III goes on to expand on statement IV. Hence Statements II, IV, and III forms a mandatory pair.

Statement I say that on paper, Stack seems easy; however, in real life, the implementation of Stack is difficult. The premise that Stack seems easy could be reached only after explaining the basics of Stack. So Statement I is the last statement.

The correct order is II, IV, III, and I.