Getting Started with CodeIgniter 4 PreAlpha 1

Hi.. I recently started my personal project in Ci 3.
I target it to work with php 7. I just want to know that is there upgrade from 3 to 4 possible.
If yes. which point i have to take care. while developing in CI3.???

1st - PHP 7.1 ? is almost but not yet in Final stage..
Do you really wan't to write something based on completely new PHP version which won't be found over any host services in next months? 
Does it worth?

2nd - CI 4 is still far away but it's already known that it will be complete rewrite of CI 3 ..
So migration will be impossible without rewriting your app.

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