4 in. PVC vs. 3 in PVC

I have a house that is 70 years old. As far as I can tell this is the original plumbing. I am remodeling the kitchen which has exposed the plumbing for the upstairs bathroom and it appears that much of it should be replaced. A 4 inch cast iron pipe runs up to the toilet then becomes a lead pipe. If I replace part of the line from the toilet to the main, should I use 3 in. PVC, or do I continue with 4 in. PVC from the hub. I am looking for what would be the benefits of 4 in. PVC vs. 3 in. PVC. The obvious benefit of the 3 in. PVC is that it fits nicely inside a 2x4 wall and the back of my kitchen counter top could be made straight.

Thank you,

David

Check codes in your local area; in my area where all the older homes were 4" they can be changed and reduced to 3".

Codes changed in my area a long time ago and you can carry 36 DFU's on 3".

If you have 4 toilets in your home if this was new construction, 4" would have to be brought to that first toilet, the rest in 3".

pvc

You can't reduce the 4" to 3" and then increase it again, but if you change everything above that 4" to 3" that would be an approved installation.