When any program runs, in any virtual-memory-based operating system (such as Unix or Linux or OS/X or Windows or IBM MVS or .. .. ..) it has a perspective of "main memory" that
never matches the physical reality.
Each program ..
every program .. "sees" no other program other than itself. "All of memory, as far as it can see," is its own
private play-pen.
The
reality, known
only to the operating system, is that "memory is, in fact, an intensely-
shared resource." But the only part of the system which can actually know or appreciate that fact
is "the operating system," because only the operating system is ever able to know how
physical memory is actually being allocated.
Every user-land program is (entirely without its knowledge or consent...) obliged to view "memory" through a hall-of-mirrors.
And what
is this "view, as seen by a user-land program, through this 'hall of mirrors' that you speak of?" It is that "'memory' is no larger than 4 gigabytes, and it contains exactly what
you think it does, and nothing more." Period.
That is what your textbook is referring to.
Yes, indeed... that
illusion, artfully pretending to be (and, from the perspective of the user-land program, indeed,
being!) "reality."
Never mind 
what "the
physical reality" is! It does not matter, after all, because a user-land program will never knows anything about what "the physical reality" might be: nor does it care.