Yes that's right. As for 4+ extra attack, well it's 1+(0.5+0.25+0.125+0.0625)= 1.9375 . 1.9375÷4= 0.484375 or ×1.484375 DMG increase aka 48.4375% MH DMG increase.
It does look like in the formula that if you use 2 Extra Attack, DMG is increased but 4 extra attack makes up for it by adding 4 extra attacks (as the name implies). Each extra attack apparently has 50% chance per each and DMG per each so less DMG per each.
Of course I did find the extra attack formula from somewhere in forums but from my own testing, 4+ Extra attack is indeed better even if the DMG numbers don't show it and 2+ extra attack shows similar DMG numbers to 4+ extra attack , though 2 less hits.
Although it's probably way more MH% than I can imagine. This is one of the issues where even I can't agree which is correct or not.
But I'd say based on damage numbers I've seen, 4 extra attack will not ruin the numbers but I noticed extra bits and faster take downs of enemies so 4+ Extra Attack is a great combo with 4+ Multi Attack. Extra Attack is like a multi attack but a 50% chance to come so chances are, it deals similar DMG to multi attack.
After removing extra attack, a significant DMG decrease in MH% , especially in my Rogue Discordance Chakram orb build from arena PvP build Will of the Force and Seasonal Maidens' build. It used to be the time when 2+ extra attack was on zenith stone instead of 4 in one slot so the general consensus was that only spend 1 slot on extra attack 2+. Now that it's 4+, you cannot go wrong really.
Extra attack and Multi attack are most definitely worth it in Primary skills and if Discordance Equipped, secondary skills. Even if it doesn't show extra attacks or multiple attacks, those attacks get converted into extra MH% DMG or so. 4x multi attack being 3× MH% DMG and 4x Extra attack having chance to give over 50% increase MH% and 4 of them .
Edit: Yep, when removing extra attack, I noticed a 50% DMG decrease overall so that extra attack definitely helps. Its a chance to deal additional attacks and each extra attack means a chance to deal X amount of additional attacks.
Sounds like 50% chance to deal 4 additional attacks where as Multi Attack is 100% chance to deal 4 additional attacks at 4+ Multi attack.
The 50% chance I saw from old description in Extra Attack before that changed. Extra Attack is like adding another Multi attack with a chance of triggering up to 4 times I guess...