You're thinking about it upside-down.
Your logic is start with a 400A panel, and then what can you put on it? Actually it works the other way round.
You start with the characteristics of your house, and major appliances... and punch that into a residential load calculation. The load calculation takes into account continuous use, equipment's ability to tolerate short overloads, and the tendency to not run all appliances full-out at the same time. That pops out a number like 274 amps, so you get a service that is larger than that, leading you to a 400A service since that's what that power company offers.
If you were using the industrial charts for continuous loads, then you derate any continuous load by 125%. So you claim to have 400A of continuous loads, OK that's 500A derated and you need a 500A service. 600A is the next size up the power company will offer, so that's what you get.
So there is no circumstance where you will have 400A of continuous loads and expect a 400A service to be involved in that.
A 400A main breaker and main panel? If cost is no object...
Imagine someone is looking for a job in this market, and they get told "we're working from home, so you'll need your own PC". They've never bought a PC before. They get sold an $8000 Alienware gaming rig with dual 3090s, and don't think anything of it, because they have no idea what PCs are supposed to cost.
The same thing can happen to novices who get "target fixation" on obtaining a 400A main breaker and panel. Yes, with enough persistence you will find vendors cheerfully willing to take $2000 for a 400A main breaker and $3000 for a 400A rated load center/service panel, all out of the industrial catalog with prices to match.
I would say this is "not common at all" owing to the extreme cost compared to the normal method.
Dual 200A panels. Cheap and simple.
Prior to NEC 2020, the typical 400A installation was a meter pan lugged for dual 250 kcmil lugs ($200), feeding two 200A main-breaker panels ($150 each) via very short, unfused service entrance wires. Through the wall, no further.
Note that this common form of 400A meter pan cannot be used any other way, since the lugs cannot accept the 600 kcmil wire required for a 400A panel, nor is it designed nor approved for paralleling dual 250 kcmil wire.
In NEC 2020 installations, that becomes a 400A meter-main ($300) which contains one 200A breaker with a slot for a second (+$150), which then feeds two 200A+ rated main-lug subpanels ($150).
Simple, straightforward, approved everyday, and uses kit stocked at local stores.
Each panel must have its own local load calculation which must fall within the rating of the panel. So you can't load them 250A and 150A, because panels with 250A bus rating are uncommon. 225A is common.