Hey, GL! I got my 75 GALLON refugium in my basement fish room so I see it all the time. And I can personally and without any doubt whatsoever confirm that critters can survive the trip through UV, past skimmer, through pump, up pipes and out the return nozzle unscathed (or, at least, still alive).
I know this because I saw with my own eyes a little, miny brittle star (the ones with the spindly legs, not the asterina sp. that sit on the glass), pass through the overflow on the fuge and into the sump. 2 hrs later, looking at the display, I saw it pop out the nozzle! He bounced down the glass to the sandbed, and scurried underneath. 5min later he wiggled a leg out, looked around to make sure the radiant wrasse was nowhere in sight, and scurried to the rockwork never to be seen again. I have to assume he has taken up residence inside the large, zoanthid-covered fiji rock into which he disappeared.
Large is an issue of water volume, of course, the larger the better. More water makes for more stable environment and all. I think the 4gal thing is small. Someone once said to me that a fuge that is less than 15% the volume of the main will not grow enough macro to be able to outcompete undesireable algae in the main ... that was MY motivation for adding a fuge, not so much the "pod farm" aspect, though the two go hand in hand (pods eat waste, bad algae doesn't grow). Then again, large not a goo didea if you are hanging it off the edge of a 55gal tank! Those weight issues are obviously relevant.
But a fuge buried in a cabinet never to be seen? Have to agree with GL. My favorite part of the hobby is watching the pods, worms, and other critters. What a waste never to be able to see them in their spawning environs!