As far as I know, 400 watt E-Ballasts are not available.
If you were to go with a 250watt E-ballast you would save money on your lighting bill. Most likely your current ballast is running the bulb higher than 400watts.
E-ballasts will also extended bulb life, saving you money there also.
There are also a few bulbs that actually run better on E-ballasts.
I use HQI and E-ballasts.
I have 2 VHO's and 2 MH's on E-ballast, and 1 MH on HQI.
You really cant tell the difference between the ballasts, but the HQI puts out more PAR.
What is your current lighting setup like?
Give me info about bulbs and ballast so I can find you something comparable or better.