How to Move Already Written CodeIgniter Code to Kohana?
See Kohana's documentation - Migrating from CodeIgniter.
A smart fellow from #kohana IRC has informed me that a simple migration no longer exists. Kohana has changed substantially since forking from Codeigniter. They are very different frameworks now, so you would have to rewrite much of your code.
CodeIgniter works well on PHP5.
It is backwards compatible to PHP4, but there is no need to run it on PHP4.
What part of PHP5 is your team not able to use in CodeIgniter?