New attic drain into existing

I have a 4" stack with a bathroom on the first floor draining into it. I want to add a bathroom in what used to be the attic - directly above the existing first floor bathroom.

Can I tie in a new 3" line from the new bathroom into the existing stack "above" the first floor toilet without a separate vent for the first floor toilet? Or do I need to bust out concrete and connect the new system "below" the existing first floor bath? The new addition will have it's own vent system...

If possible/necessary - I have plenty of room to add a vent line right above the existing toilet - between the existing toilet and the planned addition (4X4X3 Y)....

By UPC, and probably other codes, you cannot turn that VENT stack from the first floor toilet into a WET VENT by draining waste into it. What you can do is use that 4" as your WASTE STACK, draining the new second floor toilet into it, and tie a separate vent fromt the first floor, and tie it back into the 4", 42" above the second floor level. No need for a new roof penetration.

That is totally do-able. If I read you correctly - I can add a 2" vent line above the existing toilet (is 2" big enough? - I'm running out of room) - and tie it back into the main stack just below the roofline (assuming the 42" rule - if I don't have 42" - I can run the first floor toilet vent through the roof (no big problem).... The balance of the first floor (tub/sink) have a separate vent of their own....

Any upstairs plumbing will need to tie in below the connections to all of the lower fixtures.
You will need to cut concrete.

ouch... I figured as much... Back to work... thanks for the advice....

Well, a few hours later - cutting/splicing/pounding/sweating/bleeding - and the new DWV is in - and even a layman like myself can tell it's proper.... Thanks Terry

quote; Well, a few hours later - cutting/splicing/pounding/sweating/bleeding - and the new DWV is in - and even a layman like myself can tell it's proper.... Thanks Terry

Where have I heard THAT before. I do not think I have ever heard a DIYer or handyman say, "Wow, I sure did a lousy job this time." Give us a picture of what you did and WE will tell you if you did a "proper" job.