A chunk for can only be created on a keyframe, it's recommended that you have a keyframe every 1 or 2 seconds.
If the keyframe rate stays the same you will need to change the cupertinoChunkDurationTarget to "5000"
If the keyframe interval was 1 or 2 seconds it could stay at "4000" without issue.
Generally the stream will have around 25 frames per second and needs to be H.264 video with mp3/AAC audio.
What is the bitrate of the stream you're trying to play using the iOS device?