I would like to know if going above a 400cc implant will cause problems?

Thank you for your question. Generally speaking, the larger the implant, the greater the chance for complications in the long term. While 400cc is not huge, it is on the larger side. Many women are very successful with an implant in that size range. Silicone vs saline, placement, and type of implant will all be factors in selecting what is right for you. Find a Board Certified Plastic Surgeon with experience in breast surgery and make an appointment to be evaluated in person. Best wishes. 

Thank you for the question. In regards to specific size/profile of breast implant, despite good intentions, online consultants will not be able to provide you  with  meaningful specific advice.  Ultimately, in person consultation with well experienced board-certified plastic surgeons will be necessary. Careful measurements, dimensional planning, and careful communication will be key.
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.  2.  Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or breast implant size/type/profile will most likely help achieve your goals.
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "C or D cup" etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.  The use of computer imaging may be very helpful during this communication phase.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery.  The use of temporary intraoperative sizers, viewing the patient's breasts in the upright and supine position, are very helpful during the breast implant selection process.
I hope this, and the attached link, helps.

Photographs and an in person examination is needed.However studies have shown that breast implants larger than 500 cc can cause significant problems such as capsular contraction.Since your concern is of back pain that and is a new dimension to the problem.  It will depend on the size of your breast and base width and the look that you want to achieve.  During your consultation you'll undergo a breast implant sizing system evaluation during which various sized implants were placed into your bra and you may look to see how you appear with that size implant.However considering you're concerned about back pain problems if your breast measurements are appropriate for this size, I would probably recommend implants in the 250 cc range and no larger than 300. The presence of implants does affect the lower shoulder muscles of the back and less volume would hopefully impact to these muscles less.