If this is a LabCorp DOT type test it is just a normal type piss test.I've done probably 20 over the last 8 years.Nothing special,thye don't watch you pee,they check the temp,it'll be a split specimen.
If you do fail it will be up to your employer whether you get to get tested again or not.The lab don't care but they don't do it for free.

If you've been clean for 4 weeks you'll be fine.I'm a big guy 6'2" 280 lbs and it takes me about 2 to 3 weeks to get clean when I've been smoking heavily.