Expected plot didn't come out!
7 is a very small number of taps and you're expecting very sharp cutoff. Also, the cutoff frequency needs to be normalized such that 1.0 is half of the sample rate. Try a much larger order. e.g.

[b,a] = fir1(300,4/fs/2);


Also beware that the beginning of the filtered output is going to have a big transient so you might try a longer waveform.