What is the maximum frequency that can be captured by a given sampling rate?
The maximum frequency is also limited by how long you sample at a given sample rate.  If you only sample for a finite length of time at 8000 sps, the bandwidth will be less than 4kHz.  A 4000 Hz signal (or any integer multiple thereof) will be ambiguous, or simply disappear.  

A signal somewhat below the Nyquist frequency (depending on how long you sample at a given S/N or accuracy) can be "captured", as well as any signals between two successive integer multiples of Fs/2 given a-priori knowledge of no aliasing from any other spectra inside any other folds.