To predict what the new Touch 4G will be like you have to realize that Apple has two problems, a marketing one and a technical one.
First of all the marketing issue is the iPod line overall is in decline with the exception of the Touch, and Apple has basically ignored the Touch in every update since gen1 because there was simply no competition. The threat now is that with Android becoming a mature OS, and one which will be soon appear in iPad clones, it would be dead simple to use Android in a Touch like device with all the bells and whistles the Touch has been missing. No iTunes you say, well the Android Store (and its apps) is growing fast, and if the new device can play all music formats who needs iTunes.
So expect a drastic pruning of the iPod line in September, and a major update of the Touch if Apple intends to forstall a host of Touch competitors in the next 12 months.
Now the technical issue...Apple likes the form factor of the Touch and with the new iPhone 4G there is now a good visual diferentation between the two, however to add cameras, the A4 processor, retina display, gyro, etc, and maintain the battery life of the current Touch they have a problem. There is NO room in the current Touch for all this and a larger battery and the A4, retina display at al will suck way more juice, so what do they do...use the 3G(S) case that already has accommodations for a camera plus lots of room for other things including a big new battery. The Iphone 3GS is set to go away soon anyhow so then Ipod Touch will look different from the iPhone.
So will we see a redone Touch this September, absolutely, and I expect the Touch may even get a 3/4G connection over time making it a pocket iPad.