The amplitude appears to be correct. You need to define the frequency axis to get the frequencies to be correct:
final_point=3;
time=1:0.01:final_point;
omega=2*pi*4;
y1=2.5*sin(omega*time);
% %FFT
Fs = 1/mean(diff(time)); % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
Fv = linspace(0, 1, numel(y1)/2+1)*Fn; % Frequency Vector
Iv = 1:numel(Fv); % Index Vector
acc_inpt=y1;
n_fft=length(acc_inpt);
ff=fft(acc_inpt,n_fft);
num_points=n_fft;
ff=ff/(num_points/2);
figure;
plot(Fv, abs(ff(Iv)))
That plots the peak at 4 cycles/time unit, that appears to be what you want to do.
