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

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.

Best to have a consultation with a board certified plastic surgeon. Be sure to tell them about your back problems. During your consultation, the surgeon will evaluate your body type and breast dimensions. In addition, many surgeons use sizer implants to help determine the optimal breast implant size for a given patient. In doing all of this, you should avoid implants which are too large for you and exacerbate your back problems.

are certainly understandable in your situation.  You should invest in a consultation with a local plastic surgeon (and there are plenty in Reno) to review your goals and expectations and appreciate what you are desiring visually as they sometimes don't match.  Your surgeon should help you select the appropriate size for you.  And if external sizers are employed, even easier to 'see' the potential result.