CI4 sessions still not being deleted

Using 1/1000 can result it quite a large build-up of old sessions. Try these settings for a while to see if things get cleaned up.

Code:

session.gc_probability = 1
session.gc_divisor = 2
session.gc_maxlifetime = 1440

Those values are only temporary to see if GC will happen. If the extreme 1/2 doesn't work then you can be certain that something is wrong. If it does work you can settle on a more reasonable set of values.

Code:

session.gc_probability = 1
session.gc_divisor = 100

The php/sessionclean you are seeing in the log might be a CRON job that is scanning some directory other than the one you have set up for CodeIgniter to use. In other words, it's not directly related to PHP/SESSION garbage collection.