Upgrade CI3 to CI4, ORM, other...

Answer to #5 ... CI4 uses both the system and the vendor folders.
If you do a manual install, you end up with system/ for CI and vendor/ for dependencies.
If you do a composer install, then CI's system folder is inside vendor/, along with the other dependencies.

Re #2 ... there isn't an ORM specific to or known to work better with CI4.
However, we expect that many of the benefits that developers look to an "ORM" for will be addressed by the new Model and Entity classes.