You will need to verify for your area, but here I know a generator transfer switch has to have the same rating as your incoming service since both power sources go through it.
That said, if your incoming 400 amp service splits into two 200 amp service panels, you might get away with using a 200 amp transfer switch for each the individual sub-buses.
From what I remember when I had my standby generator installed, a 200 amp auto transfer switch ran about $1000, but a manual one ran about $400.
That was probably 6-8 years ago so the prices are probably somewhat higher now, but from what I saw I think two 200 amp switches would be cheaper than one 400 amp switch.
If you don't need both sub buses, you could just connect to one of them and only need one transfer switch.
I'd definitely talk to your electric company. They will probably be a better source for information on what would be legal/required.