I bought a front glass only and the company I bought it from (iRepair) had no instructions or warnings on the replacements. I am pretty good at small repairs and have done Apple hard drives on early macbooks.
The glass and digitizer are not separable! I used a quality heat gun and the amount of heat to soften that much adhesive is excessive. The perimeter adhesive was no problem as it is a thin line.
I do not believe that the glass only can be done on a Gen4 touch.