My son let me have his old 4S after getting a new 6S. Actually, I think the grandkiddos inherited this one before me until my son passed the 5S to them and this one on to me. Still, it worked fine and had been taken very good care of, but the battery had already done yeoman duty. A charge didn't last long, so before I got another SIMM card for another carrier I wanted to renew the battery.
In a word - Great!
Looking over the how-to on Ifixit's web page convinced me that this was the way to go. Ordered the kit with tools and a few days later I had everything in hand. Today I sat down to do the deed. It was over almost before I really got settled down. It's that easy. The hardest part for me was getting these old eyes to see clearly enough for those itty bitty screws. Man, those things are tiny! I like that the tools in the kit were magnetic - this made it really easy to keep hold of those little screws. I used a piece of masking tape to lay the screws on so they didn't get away from me. I lucked out with the little grounding strap as it stayed in place by itself as I pried the battery and clip up. I did find that it was easier to install the battery attachment before trying to fold up the ribbon cable and insert the battery into final position. After popping the battery into position, the back slid into place easily. I finished up by inserting the two itty bitty screws (I reused the star-headed fasteners since I believe that head is far easier to maintain in good condition than any phillips head screw). All of a sudden I was through! I doubt it took over 15 minutes and some of that was spent reading the instructions and checking pictures.
The new battery had about a half charge so I hooked up my ear buds, switched on iTunes, and went to my workshop to do other chores while I ran the battery down to the 10% lever per instructions. It's on the charger right now as I type this, getting its first full charge.
This truely was a easy fix. You'd have to be a real ham-hand to mess it up.
To ease lining up the screw holes on the battery connector, I didn't try to fold the ribbon cable first. I just laid the battery roughly in position, then fastened the connection in place. Then I folded the ribbon cable as I did the final insertion of the battery. Using that method I didn't have to worry about getting the ribbon cable folded just right for everything to line up.
Second, I made a small sticky-side-out loop of masking tape and stuck it in the bottom of small paper bowl.(my anti-static tray). I placed those small screws on that loop and didn't have to worry about them getting away if I bumped the bowl or whatever.