Is it normal to feel nauseous when cleaning breast augmentation scars for the first time? 4 weeks post-op.

Yes, what you are describing is vasovagal syncope or fainting in response to a disturbing experience.  Many people are susceptible to this especially when seeing a surgical incision on their own body.  It is important to avoid a fall thus when cleansing the incisions I would urge to sit down when doing this. Of course if this problem continues you should see your surgeon for exam and reassurance.