Polyphase filter notation
Hk are modulations of the low pass filter (band pass instead of low pass).

$$
(W_{4}^{k}) = e^{-2j\pi k /4}.
$$
For $z = e^{j\omega}$: $$H(W_{4}^{k} z) = H(e^{j(\omega-2\pi k/4)})$$
This means that the filters $H_k$ are shifted in frequency- these are the band pass filters you want to get using your filter bank.
For $k=0$ $H_0$ will pass the frequencies $[\frac{-\pi}{4} \frac{\pi}{4}]$, for $k=1,[\frac{\pi}{4} \frac{3\pi}{4}]$ etc..

In the DFT filter bank scheme, 
$y_0[m]$ are the outputs from $H_0$ , $y_1[n]$ are the outputs from $H_1$ and so on.