That makes absolutely no sense. Perhaps you misunderstood what I typed. No harm. This is what USB pass-through is:
[h=1]USB Passthrough Battery[/h]USB passthrough battery is easily charged with an USB cable. Anywhere there is the USB port with power supply, you can enjoy vaping, without considering if the battery can be powered on or not. You may like these electronic cigarette batteries, such as Joyetech USB battery, VapeOnly vPulse/flashlight USB battery or Kangertech EVOD USB battery.
No, I meant what I said. I know what USB passthrough is. You wouldn't want USB passthrough on most regulated mods, and no previous IPV series mods have had passthrough. USB passthrough is limited by the USB input, which on most USB ports is 1A, and as Croak pointed out on most mods they are .5A. Example, a .5ohm subohm tank at 3.7v would be around 7.4A, see why that's a bad idea? The definition you gave is spot on, however that isn't what you were talking about doing if you think about it.
What you are referring to could be more accurately described as charging while vaping. Which has worked on all the IPV series mods I've tried as long as the batteries had enough charge to do what i was asking of it. Well with the exception of the ipv3 which I never tried charging.
Now all of this is moot, because the ipv4 doesn't even have a USB charging port, it is for firmware updates, so obviously it won't have USB passthrough. The manual clearly states the charging option will be done with a 9V "charging cable". It also doesn't say one way or the other if you can/should vape while charging. I wouldn't do it, doesn't seem like a good idea to me to be charging batteries in series and vaping on it, but I'm sure some people will try it, and it will probably work fine for them.