Why 400ft AGL !
 

During engine failure/fire after V1, all procedures always refer to "no action below 400ft AGL" (except Gear Up , and some case manual feather of the prop)
my question is why 400ft ?
I heard that all A/Cs were certified to have the gear before 400ft AGL, which is also the start of the 2nd segment.
Any truth to that ? Any other explications ? Feel free to help me understanding this.

Cheers

Well let's see, if you have a engine fire, fail, surge, stall the one thing you need is height. Remember Aviate, Navigate, Communicate.
400 feet allows you to settle the aircraft down and fly, remember the further you are away from the brown, the better.
You should never ever rush things, this is even more true when you have a potential life threatening situation like the above.
Hope this helps

Any truth to the fact that A/Cs need to be certified to have the gear up before 400ft AGL ?

The above explains why have a height below which you don't do anything other than fly.

Your question contains part of the answer as to why the specific value of 400ft exists. I believe it does indeed relate to the definitions of the segments of the takeoff in the early regs (CAR4b, BCARs, whatever) where 400ft was an assumed height below which the configuration was essentially fixed. I think there was just a tendency to use the same height in abnormal procedures as in the performance definitions, for simplicity, and because there is no real objective reason for 400ft compared to, say, 300ft.

For example, the modern takeoff flight path rules include:
It's just a question of digging back to find when that was first introduced. CAR4b.116 does NOT have a segment break at 400ft. So it's not that old then.