The IV-4 is a very good mech and Ash is a very good player, but it seems like the OP and several commentators are confused about the connection between a mech's tonnage and its hitpoints. I can help with that.
I will use my own IV-4 as a reference, since I don't know exactly how Ash builds his. You can see it here:
https://mech.nav-alp...a6a8520_QKD-IV4
I will then recreate the mech using the build rules for the Battletech board game. The result can be seen here:
https://i.imgur.com/YMEcuUi.png
It is half of a ton overweight since the construction rules do not actually allow for an XL290. The flexibility in engine choice amounts for much of how highly optimized MWO mech designs can be. It also has half of the armour and structure, since MWO doubles our hit points in order to make games last a bit longer.
Some of the key numbers are here:
The armour takes 10 tons and the rest is equipment. So the hit points of the mech count for 13 tons of the 60 ton machine. That 13 tons is all of what the paperdoll and the percentage value in the HUD track.
Your mech may still be operational with just one point of structure remaining in center torso, head and one leg. Add one point for each side torso for my XL build or one torso for Ash's LFE build. Equipment is damaged trough critical hits, which is not directly connected to the amount of damage that counts against your armour and structure hit points.
Damaging the structure is likely to be associated with critical hits if you are not very unlucky, but you can still end up dealing several critical hits with very little actual damage or no critical hits at all with enough damage to kill a mech several times over. The underlying game mechanics for critical hits are different for the board game and for MWO, but the end result is similar. Typical way for a mech to be destroyed in the board game is for the engine to take several critical hits, but MWO does not allow for engine crits, which increases mech survivability further in addition to our doubled hit points.
Ash mentioned crit padding, which is the practice of having several vulnerable pieces of equipment in the same locations on your mech. When you score a critical hit on that location, there is a chance it counts against one of the less important pieces and leaves your weapon intact. This is a relevant part of mech optimization in both MWO and the board game, though engine counts in the board game and does not count in MWO.
Edited by Gagis, 22 June 2019 - 01:31 AM.