Just make both cleanouts 4", and run 4" pipe to the clay line. The one towards the house will seldom, IF EVER, be used so it being oversized is not a factor. Use the same dirt to backfill. Before inspection, you have to pack the dirt around, and under, the pipe about half way up, it is called "shading the pipe". If you are in Scottsdale, or a few other cities, they REQUIRE that the water lines be enclosed with insulation for protection and heat retention, but then you can use the same soil for backfilling.