What size sewer line???

current home is approx. 35-40yrs old slab construction and does not have a cleanout installed within 5' after it leaves the slab.
I dug up the line and see it as hub/spigot cast iron(checked with magnet), but it measures 5" O.D. exactly!??
I want to cut in a wye but todays modern fittings would leave a lip on the I.D. (ie.,4" Wye)
I'm really confused as to:
1. what size line is this? ( I checked a 4" Heavy Wall, but it has a smaller O.D.)
2. How to accomplish what I need to do.

Check the cast iron chart on this page: http://www.gizmology.net/pipe.htm

From this chart it appears that this could be Class B(5" OD, 4.12" ID), C or D cast iron pipe , but the problem is matching the ID with
the fittings you can purchase today. This would mean approx. 1/8" lip on a 4" wye (3.94" ID) that I can get from a plumbing supply shop.
-This is being done because last year the township cut in a clean out at the curb line and the line backed up a few months later. Now I'm
realizing that the same issue probably exists there and I thought I would be able to put a smooth transition clean out at the house (instead of
having to pull a toilet to run a snake).