Friend purchased a 2 family deweling in hopes of making repairs and then resell the home. Me, who likes to think he can be a plumber is going to help out where I can. Noticed that in this old home there are zero vents, all the drains run into the 4" cast main drain. Should I keep it that way or run 2" vents for the two kitchen sinks and also do the same for the two bathroom tub/shower and sink? And why dry vent at all if this system works?

I would suggest that you keep the existing plumbing vents the way they are.

For newly added plumbing fixtures, run new, separate 1 1/2" and 2" vents, connect them in the attic and penetrate roof with one / two vents.

Friend purchased a 2 family deweling in hopes of making repairs and then resell the home. Me, who likes to think he can be a plumber is going to help out where I can. Noticed that in this old home there are zero vents, all the drains run into the 4" cast main drain. Should I keep it that way or run 2" vents for the two kitchen sinks and also do the same for the two bathroom tub/shower and sink? And why dry vent at all if this system works?

I'm a great believer in, "If it ain't broke don't fix it". Back when I broke into plumbing most of the older homes were unvented and did just fine.
Milo gave you excellent advice. Listen to him. Good luck, Tom

I'm not adding anything new, so I'll run with the existing layout. Running a dry vent won't be a problem for me if it is necessary, and adds a better drainaige system for the dwelling?

Please let me thank both of you for your interest and HELP, taking your time to help others that you never will come into contact with is without a doubt in MY mind is the Lord's work

Thank you for your words of kindness. If you need any other help in the future, please, don't hesitate to ask.

Good luck with your project. Milo

Please let me thank both of you for your interest and HELP, taking your time to help others that you never will come into contact with is without a doubt in MY mind is the Lord's work Along with Milo I thank you for your comments.
We try to pass along some of the knowledge that we've picked up out in the field. I don't know how much a plumber thinks about " The Lord" when he's up on someone's roof snaking a sewer but the thought is nice. I hope your holidays were happy ones! Tom

Along with Milo I thank you for your comments.
We try to pass along some of the knowledge that we've picked up out in the field. I dunno how much a plumber thinks about " The Lord" when he's up on someones roof snaking a sewer but the thought is nice. I hope your holidays were happy ones! Tom

A follow up, while inspecting the 4" cast stack we found several cracks, and leaks. So we cut it out and replaced from the attic to the cellar floor. Also we gutted both bathrooms. These rooms one on the first floor and another right over it on the second floor. So now I will dry vent also. Question is can both the tub and sink share the same vent and drain? Hows 1 & 1/2 inch for the vent and 2" for the drain? It's a small room maybe 7' X 7'.

Question is can both the tub and sink share the same vent and drain? Hows 1 & 1/2 inch for the vent and 2" for the drain? Its a small room maybe 7' X 7'.
Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic.. The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes to make sure you're not in the excluded 10 percent.. The vent off the lavatory may be run out the roof or revented back into a dry vent in the attic or if you're reventing back into a fixtures dry vent you must make your connection at least 6 inches over that fixtures flood rim. In other words you can't discharge the second floor group into the sane stack you're venting the first floor with. 2" is fine for the lavatory and tub drain but I would vent the group with 2". Good luck and thank you for rating my answer. Tom