Isn't BI old, and it's not used that much these days ? so , do i have to learn chapter 4 and 5 or not ? (i think it's a waste of time)

You don't have to learn chapters 4 or 5. BI is used more than you might think, but you'll be fine not learning it I suppose.

Until BI is removed from official Blender releases, the Blender Foundation still deems it relevant.