I'm ready to build a sunken tub like I had as a kid in my country,, now 4inch slab entire bathroom has floor removed 8feet by six feet, will place 2by 6 by 60inch as a temp boarding for the pour.. top of board flush w/floor surface so floor of tub will be 5 and half deep... sides will be under slab 12inch side ways and suported by 3 on each side five eigths threaded rods. This is for shower only 60 by 32 in Rest of floor will be at same level as rest of floor .question is should I place the drain mat under slab? To connect to drain lower portion it will be a 4inch slab..? concrete absorbs water...

The Water Proofing Mat Needs To Be Ontop Of The Slab Unless Your Not Putting Tile Over Top Of Your Concrete Slab.
If Your Just Doing A Slab And It Will Be Your Finished Floor Then Lay The Mat Underneath Your Slab

Thks I think that way too , to lay the liner under slab,, like a vapour barrier ,but I'm not sure if I have to connect the liner to the lowr part of drain ,in case there is water seepage of 4inches penetration..