The cheaper screens sometimes are more frail, but in the 98% of the cases the problem is the correct placement of the screen on the device.
The screen of the ipod 4g, is glued and supported on the back aluminum case. If the case it is not straight due to a drop or other, or if the old glue of the old display it isn't removed completely when the new lcd was mounted, all seems to be fine, but when you press the screen on every day use the force is concentrated on this non straight or non cleaned points and from this points start the cracks.
Remember that this glass, is non-scratchable because is hard, but at the same time, it is very rigid, and non elastic, so it have to has mounted on a very clear and plate surface.