I'm probably trying to cram too much into my TV cabinet but is it OK to stack a PS3 Slim on top of a PS4 or would that cause cooling / ventilation issues? Only one would be switched on at a time.
As long as you don't block the ventilation, there is virtually no difference with putting the consoles on another surface.
The ventilation of the PlayStation 4 is situated here:
The ventilation of the PS3 Slim can be found here:
on the front side, under the recessed lip, is a vent that runs across the whole front on the system. On the sides is a recessed vents that cover the depth of the system, as the fans kick on, they suck in air as the push it out the back.
You can safely put them on top of each others as long as the back of them is free of any obstacles for proper air circulation.
Both consoles have their primary air outlets on the back, so it's safe to stack them as long as you don't put too much extra weight on them. I've stacked three consoles with no issues for several years now. Just make sure there is enough room for fresh air at the back.