Which of the following explains how to obtain the graph of $y=4^x-3$ from the graph of $y=4^x$?

(a) Shift the graph of $y=4^x$ up 3 units.
(b) Shift the graph of $y=4^x$ down 3 units.
(c) Shift the graph of $y=4^x$ to the left 3 units.
(d) Shift the graph of $y=4^x$ to the right 3 units.