If it's switching itself back on, it may be reading the actual charge in the battery incorrectly. Try letting it completely drain the battery (so it's good and dead and won't turn on). Then give it a nice full charge and it should reset.

That is what they told be when my 4 started chewing through its battery. In my case, it never turned itself back on. It just ran out of juice quickly, so I had it replaced. Twice.

Batteries and screens can be replaced. Some other bits aren't worth it.