Yeah the wording on that one is weird. I was making genestealer cultists (who have 3 arms alot of the times) for a game and was like **** it I'll interpret it for my own sanity. I just look at it like this: Humanoid creatures come standard with 2 arms. Multiple Arms (X) represents how many extra arms they have. After all standard human NPC's don't have the Multiple Arms(2) as traits so obviously its not intended to describe standard amounts of limbs.
As far as this goes though:
That's one of those things that's kinda up to a GM/DM. Theres alot of logical arguments both ways and RPG's typically have wiggle room on their rules for the purpose of fun. Personally I might make it dependant on what items are in said arms/ if the weapons are part of the arms. Example a mutant human wielding two small swords with its extra pair vs one great weapon would make a big difference if one of the arms got chopped. Or say a tyranid loses a single arm out of its pair of scything talons. Each of these situations is a tad different in my mind. The nid as a creature narratively programmed via its DNA to use its multiple arms would probably be less affected than either of the human mutant situations. The difference between the human mutant situation is that the set holding a two handed weapon obviously cant swing anymore while the two one handed weapons is more nuanced. Could be that narratively he wasnt trained to use all 4 arms at once and that he only ever actually used 3 out of 4 at any time (a lore reason/out behind why 4 arms means 3 attacks instead of 4), so he can still make 3 attacks though because he has 3 arms. Or you could follow rules as written and say he's below 4 and therefore only gets 2 attacks now effectively making the 3rd arm useless.
I actually dealt with said hypothetical situation with my previously mentioned genestealer cultists. Some cultits had 3 arms and some had 4 so I decided that 3 arms ones didn't get an extra attack but they did get one free draw/stowing of any weapon that could be considered one handed one each turn in combat. Then they could pick to use 2 out of the 3 equipped items, or could have a basic class weapon and a single handed weapon at the same time. That way the arm isn't useless but its not granting extra attacks. And I had the cultists who were high strength with 3 arms always count as being braced for the purpose of firing heavy weapons, so long as all 3 arms were using the single heavy weapon. So if your the DM its up to you man. If not run it by your DM should you ever have it come up for the game actually and see what they say.
Also yeah some things from different systems just don't mix well sadly... best to avoid when possible and if anything use the wording from the system your in over the one your taking from to avoid confusion or balance issues.
