Why aren't there any 400V ultracapacitors?
A high-voltage super-capacitor/ultra-capacitor would be a contradiction in existing technologies. They achieve their high capacitive values by having a super thin dielectric of special materials, hence the low voltage limit.

To build a 400 volt capacitor means having a thicker 'solid' dielectric with more common materials, which 'fattens' up the size a lot.

There are large 450 volt electrolytic capacitors to 20,000 uF or more. To have a super-capacitor with the same voltage rating would be duplicating the large can-type electrolytics.

Super-capacitor/ultra-capacitors have been around a couple of decades now, mostly changing in materials to cut down on self-leakage. Graphene is the latest trend. The laws of physics and chemistry limit the size of such capacitors, or we would already have them to buy.

Engineers would love to have giant super-capacitors for cars, etc, so there is an on-going effort to reach that goal.

This is a snippet from Wikipedia:


  A supercapacitor (SC) (also electric double-layer capacitor (EDLC),
  also called supercap, ultracapacitor or Goldcap) is a high-capacity
  capacitor with capacitance values much higher than other capacitors
  (but lower voltage limits) that bridge the gap between electrolytic
  capacitors and rechargeable batteries. They typically store 10 to 100
  times more energy per unit volume or mass than electrolytic
  capacitors, can accept and deliver charge much faster than batteries,
  and tolerate many more charge and discharge cycles than rechargeable
  batteries.
  
  Supercapacitors are used in applications requiring many rapid
  charge/discharge cycles rather than long term compact energy storage:
  within cars, buses, trains, cranes and elevators, where they are used
  for regenerative braking, short-term energy storage or burst-mode
  power delivery. Smaller units are used as memory backup for static
  random-access memory (SRAM).
  
  Unlike ordinary capacitors, supercapacitors do not use the
  conventional solid dielectric, but rather, they use electrostatic
  double-layer capacitance and electrochemical pseudocapacitance, both
  of which contribute to the total capacitance of the capacitor, with a
  few differences:
  
  Electrostatic double-layer capacitors use carbon electrodes or
  derivatives with much higher electrostatic double-layer capacitance
  than electrochemical pseudocapacitance, achieving separation of charge
  in a Helmholtz double layer at the interface between the surface of a
  conductive electrode and an electrolyte. The separation of charge is
  of the order of a few ångströms (0.3–0.8 nm), much smaller than in a
  conventional capacitor.
  
  Electrochemical pseudocapacitors use metal
  oxide or conducting polymer electrodes with a high amount of
  electrochemical pseudocapacitance additional to the double-layer
  capacitance. Pseudocapacitance is achieved by Faradaic electron
  charge-transfer with redox reactions, intercalation or
  electrosorption. Hybrid capacitors, such as the lithium-ion capacitor,
  use electrodes with differing characteristics: one exhibiting mostly
  electrostatic capacitance and the other mostly electrochemical
  capacitance.