Will Toto Ultramax Clog My Cast Iron Pipes?

I have an old house with 3.5 gallon gravity flush Kohler toilets.

I also have cast iron sewer lines which are either 4 or 5 inches in diameter) not sure which).

Will I have clogging problems with the Toto Ultramax because it is not pressure assisted?

What is the best toilet for my situation?

Your cast iron drains work on gravity so no, the Ultramax wont be the cause of clogging. The pressure assist models use the pressure to get the contents of the bowl out of the toilet and into the drain where gravity then takes over to move the waste along.

I'm at 250 to the street sewer with 4" lines.

Interior is cast iron, from the home to the main it's clay tile.

It will be fine.