I have owned my S4 for about 4 weeks. I had had no problems until a week ago when my phone froze twice in a period of one hour and last night when it froze once. In all three cases, the phone was either fully charged or mostly charged. On one occasion, it was being recharged (but still was mostly charged before plugging in the recharger). The symptom was that the phone was totally non-responsive. The solution was to pull out and reinsert the battery. The phone is not rooted. I do remember seeing a message a couple of weeks or so back stating that my phone's op sys or some such subset was going to be updated. I did not pay attention to the details (my mistake) but just gave it the go-ahead. The phone is a SCH-I545 on Android 4.2.2. Kernel version is 3.4.0-690088 updated on June 13.
Obviously, the first suspect would be an app that I installed. Most (all?) of the apps I have installed in the last month are fairly benign. I cannot recall anything specifically that I recently installed (especially in the last week and a half) that would make me suspicous but I'm well aware that that doesn't prove anything. I did note the forum message on how to put the phone into safe mode but I am hesitant to do that as I really use my installed apps. Plus the problem isn't happening with very frequent regularity. Does anyone have a list of "suspicous" apps that other people have tried to track down? I didn't see anything about frozen S4s on the forum. One last comment: the problem occurred last night (Saturday) and the previous Saturday night, as I was about to go to bed. I may or may not have tried to set my phone alarm clock (I use the downloaded "Alarm Clock Plus" which I have used for years with my previous Android phone, a Droid X). I honestly cannot recall.
One other possible source of the problem is that it might be a hardware problem with the phone itself. Again, this is hard to prove when it doesn't happen with great regularity. Has anyone seen any such issues?
Suggestions and comments would be welcome.