So I'm planning a heretek character and wondering how a few things would interact. Hereteks begin with 2 free cybernetics and so I am wanting to take 2 cybernetic arms which would give my character 4 arms, would this give my character the multi armed trait and how would this interact with multiple weapon fighting? If I take the two weapon wielded trait would I be able to make 4 attacks from a single standard attack action?
Multi-armed fighting
Bionic arm, as it is mentioned in the very first sentence of its description, is used to replace limbs, not granting multiple limbs trait.
"One of the more common replacement limbs to be found" yes you can use it to replace an arm but there is no requirement to be already missing an arm.
with 4 arms total (2 biological and 2 bionic) you would be multi limbed (4) so you would get to make your main hand attack then two offhand attacks. That two comes from 4/2=2. So if you're carrying power swords in every arm, you could be making up to 3 lightening attacks as a full round action.
MULTIPLE ARMS (X)
This creature has more than one pair of arms. The exact number
of arms is indicated in parenthesis (X), so a Genestealer with
Multiple Arms (4) has four arms total. It gains a +10 bonus on
Athletics Tests involving climbing and swimming. In addition,
for each pair of arms it has—the Multiple Arms value divided
by 2—it may make an additional attack as if wielding multiple
melee weapons (the creature must either have natural weapons
or actual melee weapons to wield for this to take effect). Note,
that a creature still requires Two Weapon Wielder (Melee) to
make multiple attacks in the same turn.
"One of the more common replacement limbs to be found" yes you can use it to replace an arm but there is no requirement to be already missing an arm.
No, and you can chop your perfectly good arm to install cybernetic replica. But multiple arms? See, arms need to be attached to something, and be linked to the brain somehow. You want multiple arms - you need either a mutation or a through surgical reconstruction of the entirety of your upper half, which needs way more than just cyber-arm.
Considering that to attach a cyber arm you already need quite an invasive surgery I don't think attaching it somewhere else would be much different.
also a picture (from lathe worlds I think) for proof of concept.
Eldar homunculs are famous precisely because they can pull that sort of stuff while others do not, so the difference is huge.
The closest you can get is servo harness, and that's quite different from multiple limbs.