Breast Reduction from 500cc to 400cc?

If you really think your breast implants are too large reducing them by 40 to 100 mL doesn't make much sense. They really will be that much of a

Volume of implants much less important than dimensions.  When you reduce to 400, will this be a reduction in projection, in width, or both? How much of your breast volume is implant?  If your implants occupy half your breast volume, then a 20% reduction in implant volume (500 to 400cc) represents a 10% reduction in breast volume.  Will this be enough to make you happy?  Whether or not you will sag depends on several factors including skin elasticity, difficult to evaluate online.

In order to see a significant change in breast size, it is likely that you will need to downsize breast implants at least 100 mL. In other words, the change of 40 mL may be a waste of your time/efforts.

 Whether or not additional breast lifting will be necessary, will depend on your physical examination. As you know, if you end up with loose/"sagging" skin after the downsizing of breast implant procedure, the overlying skin “envelope” will need to be tightened. You may want to give your plastic surgeon  your consent to revise your breast lift if necessary, at the time when you undergo downsizing of breast implants.

 I hope this, and attach link, helps you as you learn more about revisionary breast surgery.

Best wishes.

Photos are important but a physical exam will be decisive. A 40 cc reduction in implant size will not make a difference. It is impossible to say whether going smaller than that will result in significant sagging. Another issue is whether the problem is size alone or the profile of the implant.

It is difficult to predict how the breast will respond with a 20% reduction in implant volume.  it may be fine, and it may not. Many factors go into this.

There is not enough info here to give a useful response. Photos are needed, and some idea of time course of your past surgery. Consider reposting with that material. I can say that I doubt 460'ies would look any different. 

All the best. 

A 40 cc reduction in volume is almost no reduction and you may barely notice the change. A 100cc reduction is more significant. Unfortunately there are far to many factors involved to be able to answer you without an examination and the specifics of the implants to be used. In general, however, if you have good elasticity to your skin and a relatively good amount of natural breast tissue you probably won't have any significant sag.