Is there a way to isolate a frequency from raw data and get the start and end indicies?

     I am running an experiment which measures the forces & torques in the X Y Z generated by a flapping wing. I am basically controlling a PWM to basically control the frequency of the flapping- I usually aim for either 4 Hz or 5 Hz. 

     I did not set up the data acquisition system- but from what I understand so far it is recording the different variables in real time domain.

     I am able to load just one of the variable at a time on to Matlab, such as the force in the X direction for that trial run. This is just the raw data though, before I have filtered it or done anything to it. I want to not only look at this raw data, but I want to use MATLAB to go through this vector (I usually collect about 90 seconds worth of data at a time, so the vector is 130000 long ), look for the longest interval in which the data is coming in at 4 Hz (with margin of error of maybe .01 Hz) and give me the indicies of the start of that interval to the end of the interval of good data.

   I am not new to matlab, but I am new to using matlab for this application. I hope that my question makes sense- and any help would be greatly appreciated.