I am connecting a 2" PVC steam/shower drain into the main 4" PVC drain under the basement floor. The floor has been opened up with the 4" PVC exposed. We will use a "Y" Tee to make the connection. We are 5 feet from the 4" main.
1) Do we need a vent?
2) What is the correct type of mechanical joint to use under ground when we cut the 4" main?

Let me run it for you. Will a permit be pulled for this job?
From the 4"main you will cut in a 4X2 wye or combination wye and eight bend. Making your slope you'll 1/4" to the foot you will pipe up next to the 2" trap where you will roll up a 2" wye or combo and run up a 2" vent pipe that will either go out the roof or revent back into a dry vent to roof.
From the 2" fitting you may connect the trap and the shower drain.
You did know that every trapped fixture HAS to be vented didn't you? Did that answer your question? If not click on bark. Good luck with your project, Tom

I always thought f you were close to the main you did not need to vent.
We are 5 ft from the 4" main and the trap will be under the concrete basement floor
Yes a permit will be issued. Need to get it right the first time.

DIY Phil: You are not saying what Plumbing Code you fall under. I work under UPC. Here, you need a vent for every plumbing fixture.

I always thought f you were close to the main you did not need to vent.
We are 5 ft from the 4" main and the trap will be under the concrete basement floor
Yes a permit will be issued. Need to get it right the first time. No matter what code you fall under you are sunk by two plumbing mandates.
1. Every fixture that has a trap MUST be vented**and**
2. You can not discharge a major fixture ( such as in your 4" main) past a unvented minor one.
Now Milo works under a stricter code,UPC then I do at SPC so I can't imaging that Milo would tell you any different since his code vents everything including the family dog.
It's for sure if you've permitted this job the inspector's going to be looking for a vent on that shower of yours. Good luck, Tom