Have you had that portion of basement tested for radon?

The whole house was tested when the radon system was installed 15 years ago.

This pipe is in the slab from the original house, the radon system now originates in a part of slab located in an addition that was put on the house before me. That is to say that the radon people determined that where the system is located now must be the best (probably lowest point?) location in the slab... I believe this pipe was from the original pre-addition construction.

If you truly don't need it one sludge hammer tap, insert some 6 mil plastic and fill it with a concrete... Solved

Did they put radon systems in in the 70’s? I think the purpose of this pipe is something different. Maybe the builders were thinking of adding a bathroom or planning on a sump pump if needed? I would take the cap off and have a peek before doing anything permanent.

You sure that isn't simply a toilet rough-in in case someone wanted to install a bathroom in this area in the future?

I'm with kudzu9 on this one. Why would someone rough in radon here? It would make sense that a toilet drain would have to be installed before the slab was placed; the supply lines could be dropped from the ceiling.

See if you can determine the schedule rating of the pipe please. If it's a 20, it shouldn't be a sewer line. I'd cap it in the least destructive manor possible.

The reason I'm convinced it probably isn't a toilet rough in is because a) there is not an accompanying rough-in for a sink... Why would you rough in a toilet and not also a sink? and b) there is no where that this pipe could come up and into the sewer main unless it had its own dedicated line to the septic... We have 3 main sewer outs in the house, one of them is about 15 feet away from this pipe and is fastened to the basement wall about 40" above the slab, there isn't anywhere visable that this pipe would come out...

RE RADON:

The original house was built in the 70s, this part of the foundation is from the original build. There was an addition made in the 90s. There was never a radon system actually installed, until we moved in in 2000 -- Out Radon system is originates on the opposite side of the house in the addition's slab and vents out through the highest point in the house. My thought is this pipe was the rough-in for a radon system and probably runs a perforated pipe underneath the slab, my thought process here is also that this is right next to a load bearing wall that supports the point that was the highest wall in the original house

Is it possible that the building permit office for your area would still have a copy of the original plans on file?

Also, the pipe says on it 4" sewer, although (and I'm definitely not a plumber) the cap on top doesn't look like a sewer cap/clean-out access point?

Had a plumber look at it, he thinks radon, but I'll pop the cap off this weekend and take a whiff... if it goes to the sewer, I guess I should smell it. I'll also give the sound test a shot. Plumber said if I can see stone at the bottom of the pipe or if the pipe makes an aggressive kink in multiple directions its definitely radon and since there's a radon system elsewhere in the slab I should be able to cut, spray foam and concrete over it... I'll follow up. Thanks for all the answers so far!

I had a sewer rough in just like that. It had no odor, but I could hear water flowing from other, connected fixtures when they were in use.

If it is a rough in that is tied to the radon mitigation under the slab, then you should be able to test this. Pull the cap and drop a piece of string down the hole. The string should flutter with the radon fan on. Turn the fan off and the string should stop moving. I have the same exact situation. Your may also see dirt in the horizontal pipe since it should be perforated to pull gas under the slab.

In the 70s there was no radon requirement and no testing for radon was done.

Radon in homes was ruled out as a health risk only in the 80s and if I remember right "indoor radon requirement program" went into effect in the late 80's early 90s when every home had to have radon testing and fitted for radon remediation so I doubt anyone even thought about that in the 70s.

Like others mentioned it could be a rough-in for a future toilet or some sort of a cleanout, or for future floor drain in the basement, etc.

Looks like a sewer lateral cleanout to me. I have one identical to that in my basement (near where my sewer exits the house) and also one in my yard but it's a newer development

Say it is a sewer access point - can't these be chopped down with a flush cap over them?

I also think it has something to do with a sewer, or maybe a sump-pump.

Does your work-out room need a built-in bench? ;-) Perhaps you could take another photo of the location in relation to your wall.