4 20 amp 117 circuits fed from a single 40 amp 234 circuit WILL NOT cause your breaker for the 40 amp circuit to trip at 50% power draw, unless you don't wire the 117 circuits correctly in the secondary panel.
The way the panel is set up, 2 of those 117 circuits end up IN SERIES with the other 2 (with the neutral transferring any imbalance back to the pole transformer) IF you wire the thing up correctly, 2 breakers on each "side".
For reference, common 234 volt service in the US (and Canada) is set up as "split phase", more widely known as a "center tapped" arrangement.
Half of the 117 circuits are set up to be on one side of the "neutral" center tap, the other half on the other side, with the WHOLE of each side being in series with the WHOLE of the other side, optimally keeping the draw on each side balanced or nearly so to reduce the current on the neutral feed line.
THEREFORE, you can pull *TWICE THE AMPERAGE* at 117 as your 234 feed provides, as half that amperage is in series with the other half.
This is not theory, this is a union-trained electrician and a VERY long time electronic tech explaining FACTS.
Why did my 100A panel trip with only 8 * 15a circuits then? 4 were on left leg and 4 on right leg. Draw was around 50A on each side, more like 45/55A measured with clamp. However this is the bit i wasn't certain about. You seem to be saying i could have used 160A @ 120V on a 100A panel as long as i balanced each side properly?