It's not. 4GB includes almost 4GB of packages that you don't need to have "bare" LaTeX.
I know, but what is in even the biggest set of optional packages to reach 4GB? Fonts? Viewers and tools? Maybe it's just the old unix fart in me, but we've had entire OSs with many GUI apps in much less than 1GB.
It isn't. That figure is more like the size of everything there is to do with LaTeX, uncompressed, than the size of LaTeX itself.