How to increase 400 V to 1600 V? Or oscillate the 400 V for a voltage multiplier?
I am using a flyback transformer to obtain 400 V DC from 120 V AC
  mains.


OK, so instead of rectifying this output in the conventional way, attach a cockcroft walton multiplier to the output like in this flyback converter: -



The above is not an exact drop-in for what you want but is intended to show that a relatively high voltage secondary on a flyback transformer can be boosted by a cockcroft walton type multiplier.