Had a problem with 3 screws that would not budge - a bit of heat from my gas solder iron and a bigger philips screwdriver solved it - replaced the 2TB WD20EADS green with a 4TB WD40EFRX red ... did an erase and seems to work fine - easy peasy ... thanks for the guide
I had problems with the fit of the mirrored back . Some of the tabs on the new back did not align with the 4 S - so rather shipping back the part I tried to make it fit - cut of 3 of the tabs that did not align and then it fitted and works . It is slightly thicker than the old one and the external body-glove cover fit is a bit of a squeeze but make do .
Seems that some of these generic part are made for a few models ans do not exactly fit . Knowing this now I probably would have gone for the real apple back than the generic one . I would also have expected that ifixit should have mentioned it .
Now for the hard drive replacement in my Mac Mini - hope that one goes easier :-)