If there is a slim I think they will have to take the power unit out. They can't shrink anything else and I remember the PS2 slim also having an external power unit.
If there is a PS4 slim and a PS4 Neo this holiday season, i think we will get them at:
PS4 Slim $300
PS4Neo $400
I wouldn't be surprised if both are bundled with GTS with the PS4Neo getting a limited edition and being at the front of all holiday marketing campaigns.