When will the iPod touch 4th gen be discontinued?

We don't know when it's going to be discontinued, but it's still sold, so probably Apple will stop selling the 4th generation iPod touch with the next generation iPod

The only 4g iPod new stock for sale now is the 16gb iPod touch. As this is an entirely new configuration, it's possible that they have decided to sell it for a while longer. In any case, only Apple knows for sure and even they may not have decided yet.