400kV is a pipe dream unless there is several feet of insulation
One of our ion implant accelerators had a DC terminal potential of +250 kV with respect to earth ground on a good, dry, day. It still occasionally arced to ground for no apparent reason. The path through air was about two meters or so. It is almost impossible to create 400 kV in air without humongous corona losses, although there are some high-voltage power transmission systems that somehow manage to cope with voltages as high as 765 to 800 kV. See graphic below and
here.
I doubt anyone visiting this forum has the wherewithal to purchase one of those transformers, but if you do
Siemens has a nice glossy blow-sure if you want to see some pretty pictures. Plan on setting aside an acre or two for installation, and also add cost for fencing, concrete pads, and perhaps a rail spur for delivery of your custom made-to-order transformer... It pays to shop around for transformers (of any size) and, often, used transformers can be purchased for half the cost of a new one. Please let us know what you are trying to
DO and perhaps someone can direct you to an appropriate supplier.