How to find fft for part of my output? I mean after removing the initial transient of my data, I need for only x(4000:5000).

 x and t are vectors of 0:0.1:5000;

n = length(x);

dt = t(end)/(n-1);

Fs=1/dt ;

NFFT = n ;

y=fft(x,NFFT) ;

f=(Fs/2)*linspace(0,1,NFFT/2 +1)*2*pi;

figure(1)

subplot(2,1,1)

plot(t(4000:5000),x(4000:5000))

figure(1)

subplot(2,1,2)

plot(f,y(1:NFFT/2 +1));

figure(3)

plot(abs(f),abs(y(1:NFFT/2+1)));

Pow = abs(y(1:NFFT/2 +1)).^2;

Pow(1) = 0;

figure(4)

plot(f,Pow)