I would consider taking the 6" over the 4" if the underlying cymbal is not much bigger than the top cymbal.
If the underlying cymbal is much bigger, the 4" will do.
Except... when you want to hit the bell of the underlying cymbal...in that case, the 4" will be too short.. So decide what you want carefully.
These stackers are great to exploit a minimal setup and still use more cymbals.
However: when you screw it on the cymbal stand, I can't tighten the underlying cymbal anymore. No more space for the nut.
That underlying cymbal just hangs loose, only with a felt for some protection, but the nut to fasten it doesn't fit anymore, so little diameter cymbals will move around more than it should, let alone what happens with big cymbals which are unsecured with a top nut.
So to secure the underlying cymbal, you'll need to use a less bulkier nut than Gibraltar uses, maybe also less bulkier felts, and the problem is solved.
(that's why you should always have a bucket of that spare stuff lying around)
Cheap, solid and wonderfully usable.