What kind of conductors and panel(s) were used for your 400 amp service? I’ve only seen 400A 1ph service done once and in that install the electrician opted to use 2 parallel runs of 200A ea and 2 200A panels.
I will need to upgrade my shop electrical service at some point to handle a 40hp RPC.
At my last house I had 400 amp service. It was 350 mcm aluminum wire to the meter base, where it split 200/200 between the house and the shop with 4/0 aluminum conductors. The county electrical inspector spec'd the wire sizes. It was a lot of runaround to speak to the correct person to get this done when dealing with FirstEnergy in northeast Ohio.