4'11", 125 lbs, 34B. Is 400 cc too big for my size? (Photo)

Thank you for your question.  Looking at the photos I do believe you are starting a great starting point and have realistic goals.  I typically don't concentrate too much on cc's I tend to tell my patients let's concentrate on cup size.  I would suggest seeing a Board Certified Plastic Surgeon and also seeing if they offer 3D imaging.  This tool helps narrow down the implant size and gives pretty realistic outcomes.  Best of luck to you. 

Many factors come into play to determine the appearance and outcome following breast augmentation. Although the volume of the implant plays a significant role, there are other factors that are equally important. It appears that your goal photograph is achievable. Make a consultation with a board certified plastic surgeon to confirm the best way to achieve this. 

There are many factors that go into implant selection.  In addition to volume, there is shape, style, and type.  The size you are considering appears to be in the right ballpark based on the photo of your "dream" outcome.  Many surgeons have sample implants you can "try on" to get an idea of how you'll appear after surgery.