I don't know much about iphones, but if they are 4 years old, your battery might be going bad. Also, you could try backing up anything important, doing a complete cleaning of the phone (regardless of storage space), and then reinstalling the system.
app is freezing and iphone is becoming very hot
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I have iphone X and iphone 8 plus. Both with 256Gb space and more than 80% of the capacity is free. They are both up to date.