Water tank? you must have a very crowded place. Geez.
I don't reccomend it. At least on the quicksilvers a LOT of the airflow comes out the bottom left, if you block that the fans speed up and it gets rather warm.
On the MDD the issue is no different, as the main vent is STILL on the bottom (for cold air, of course), which means that you'll want to have an inch or three on either side.
Besides that if you stick it in the cabinet any noise you'd loose by buffering it with the fake wood is only going to be lost when the fans kick into full blast because the machine can't get proper ventillation.
Point? you'll want it good on the back, but good on the sides. IF the table has 3" of clearance between the back of the desk itself and the wall then you might be fine. If the G4 can sit in there with at least an inch to spare on the left and right, then you're ok, if not.. then I wouldn't reccomend it.