Which PHP 5.X version should be considered as Required

I can play the broken record all day, but I think, eventually, it will get on everyone's nerves. You did not answer the question in the least:
What features in 5.5 or 5.6 are going to make CI 4 so much better than it would be if it targets 5.4?

I think it's a fairly difficult question to answer from the point of view of a framework developer, otherwise I wouldn't bother to ask. I don't think many people have a hard time justifying the use of 5.3, and I think that while 5.4 is significantly more difficult to justify as a necessity than 5.3, it's much easier to justify than 5.5 or 5.6.

Nothing is too important. You can keep and 5.2 or 5.3 as base for CI 4 its fine..

"Possible old servers" where can be uploaded a new (2016 build) app with CI 4 sounds strange as a reason to keep down line of CI to is an old version as 5.4.

BTW such servers will be in a lot worse condition to match all issues with the PHP it self and other things as ssl and etc if they are not proper upgraded. 

I have listed at the first post all difference in the version. PHP is growing in small steps if you don't want to jump often you have not to skip them as we have done that with CI 3.
Although CI 3 took too much time to be released else it was not going to be a big step next CI 4 version..
PHP 5.5 has a lot of features as generators, finally and etc..
PHP 5.6 has less new features but still there very nice new operator ... and it has a lot of improvements inside it and it should be focused as the BEST of PHP 5.

I agree that may be 5.6 may be too high line but less than 5.5 is a 100% mistake!

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