The most generic sine function you can write is
[tex] y = A\sin(\omega x + \phi) + k [/tex]
Of all these constant, [tex] A [/tex] affects the amplitude, [tex] \omega [/tex] affects the wave length, [tex] \phi [/tex] and [tex] k [/tex] cause horizontal/vertical translation.
In your case, [tex] A = 4,\quad \omega = 3,\quad \phi = k = 0 [/tex]
And we are interested in the amplitude, so we focus on [tex] A = 4 [/tex]
The amplitude of the standard sine function is 1, so when you multiply by [tex] A [/tex], the new amplitude is [tex] A [/tex] itself. In our case [tex] A = 4 [/tex], so the amplitude is 4.
