crystal ball needed for old Boston home

We are going to change the toilet. I have no idea whether the waste pipe (which is the original 1929 cast iron) is 3" or 4". Was either of these the preferred type used in the Boston area during the late 20s? Obviosuly, I will know once we get the toilet off, but since this a new activity for us & we only have a bathroom, I would like to have as much info as possible prior to making the change.

Thanks.

It doesn not make much difference for the toilet.
Toilets work with either 3" or 4" drains.
The floor flange that the closet bolts are held by are of the same size.

pipe

4" was almost the de facto size through the 50's.

Thanks. I wanted to have some replacement parts on hand, just in case.

It turned out to be a 4". We had flange replacement parts on hand but fortunately did not need them.