Ok I thought I would update you guys on what has transpired since yesterday when this thread began.
I took all of the advice to heart and did not even bother going to the city.
I started at 8 AM this morning and dug up all of the dirt in front of my porch so that I could expose the 3" waste line to 4" clay connection that was supposed to have been done when I had all of the waste lines replaced 2-1/2 years ago.
What I found was that there was a 3" cast iron pipe connected to a wad of concrete and that my 3" ABS waste line was connected to this cast iron pipe with a basic flex interconnect.
So I took the bull by the horns and cut the 3" ABS, then went under the house and cut the opposite end of the 3" pipe that went under the front porch, under the foundation footing and served the entire house.
Then went do the plumbing supply and purchased all of the items I needed to replace the 3" pipe with 4" pipe to complete the task mentioned in the original post.
I also called a local plumber who is showing up tomorrow to cut the clay pipe nice and clean for me at which point we will connect the 4" ABS I ran under the porch to the 4" clay city waste line.
I temporarily installed a 4" to 3" reducer and used a new 3" flex hub to reconnect my new 4" to the cast iron 3" so that we can shower and use the bathrooms tonight.
I then went under the house and reconnected the sink drain (4x2) and temporarily rejoined the original house lateral 3" branch to my new 4" branch (4x3) and did a flush and shower test by having my wife and kids flush the toilets and run both showers at the same time to make sure I don't end up with a flood under the house overnight.
All in all I am glad I did this because the cast iron 3" pipe that my old 3" ABS was connected to was pretty crusty inside and the more I thought about it like WestCoast said, do it right or don't do it at all.
So when this is all said and done the waste system should be bullet proof and I may even go the extra mile and run 4" up to both of the existing bathrooms and have the 3" shower and toilet drains all tie into 4" ABS so that I never have to think about it again.
I really appreciate all of your help and I can say without a doubt that this forum by far has more cordial professionals who are willing to participate in a thread than any other trade forum I have visited and I want to thank you guys for that.
Again remember that I am doing most of this work myself by financial necessity and not because I want to try and just save a few bucks.
Every hundred dollars counts with this project and the more I do the more I can basically pay myself during this writers strike which has left me unemployed for the last 2 months.
Thank you guys for taking the time to help me understand what I needed to do.