I purchased a house with an unfinished basement. I hired a freelance contractor that claimed to know plumbing. He did finish all the walls and the bathroom. The waste line runs underneath the cement floor. Previously there had been a bathroom there, so there wasn't a problem to install the toilet and the shower tube. Also there was a approximately a 4" black pipe connected to the main drainage and ran outside and up to the roof. The contractor cut the black 4" pipe from the main drain and connected it to an extractor fan on the sealing of the basement. The stinky smell comes out from the shower not from the toilet or the sink. My question is, a water trap should have been installed underneath the shower to absorbe the reflux gas? Please help.

If the smell is coming from the shower there might not be a trap assuming you use the shower,otherwise if the water in the trap evaporates it's going to smell just like there was no trap.But sounds fishy if he connected the 4" ABS pipe to a fan,because the fan can suck the trap dry in a couple of days if the shower is using the same 4" vent.If they share a vent it should have maybe have been ran to the exterior of the foundation and then up to the roof.Shine a light down the shower drain if there is standing water then there is a trap.If not run the water for a couple seconds then check again.