No air pipes in upstairs bathroom

While examining the plumbing in my upstairs bathroom I noticed there are no air pipes. It is a straight run of about five feet of 4" PVC pipe . It starts with the toilet followed by sink drain before it turns downward into the stack.
The bath drain comes into this straight pipe perpendicular.
I would want to add a an air pipe that vents through the roof. My question is does it matter where the air pipe is added? Some parts of this run are more accessible than others. I am aware that new building code says an air vent should be at every drain but that is not very doable. So where would be the most effective place to add a single pipe ?

Pictures if possible and it's wet vented. You can drain a tub and sink into a stack and vent the toilet, sink and tub through that entire stack. If its not going to outside air that's very surprising. If its 4" you should technically increase one size before you penetrate the roof, so a 4x6 coupling. At least in my area that's what we do.

And put your vent as a continuation of the stack. Where is it venting right now? It's not capped is it ? And also toilet must be the lowest connected fixture in a wet vent and anything draining beneath that toilet say first floor has to be separately vented

Thank You
From what I am reading it sounds like your recommendation is to put the air vent as close to the stack as possible and the further recommendation to bring the system up to code. Although I can appreciate the bring it up to code advice that would require far more construction and expense than I am willing to spend right now. From my perspective I am least attempting to bring it closer to code than what I started with.
I noticed the problem while looking at a vent pipe rising through the attic ceiling. The other end however was just buried, unattached in some insulation. Probably to fool house inspectors. I intend to run this pipe into the closed system mentioned above. It will at least offer some venting. It then becomes the question of where to insert it.

Thank You for your help.

Yeah just do what you can I suppose, eventually get it done properly but it'll make it better than what you have.

I would connect a 2" vent pipe to a new tee at the sink drain and extend up to to the existing roof vent that's in the attic. With the 4" waste, which is good size for a bathroom group the single vent will keep the lav and shower from siphoning..

I would connect a 2" vent pipe to a new tee at the sink drain

Click to expand...

Well thats a possibility - it would just require cutting of the sub floor in order to gain access. Some of this run can be easily accessed from the room below , however that area would be more challenging.

Can you post pictures of what you have now? I am having trouble envisioning what your system currently looks like.

It is really a simple system. Imagine a 7. The long vertical line is the stack pipe. The short horizontal line has a toilet at the furthest end and a sink drain close to the stack pipe. The bath line is a perpendicular line intersecting the short line ( between the toilet and sink drain).
That's it. By the way I have never had a problem with gas odor or drainage.