Tetra Treasure Chamber can be accessed by giving 1 Platinum Coupon to Ivan Popovichi at Coimbra Nimrod Bridge (J9). You will be warped into a room with lots of Treasure Phobitans where you can kill and loot for 3 minutes. The first room contains a warp portal to the 2nd room, which in turn contains a warp portal to the 3rd room. Each use of the warp portal requires 1 additional Platinum Coupon. This is essentially a higher version of Al Quelt Moreza Treasure Chamber.
The Lv.50 monsters in the 1st room drop Refined Ionium and Lv.52 equipment. The Lv.60 monsters in the 2nd room drop Refined Aidanium and Lv.64 equipment. The Lv.70 monsters in the 3rd room drop Bulk of Silver and Lv.72 equipment.
Although used in some quests and mid-level elite recipes, these ores are easily available in Gigante Beach or Fire Island Free Zone. So, Platinum Coupons are not really useful.
P/S: Yes, this is a very old feature of the game, but I got nothing else to write about, so meh...