Implants slipping out of pocket, falling to outsides of chest. 4 months post-op.

Natural breasts fall to the side when lying down, more so with larger breasts. Larger implants place more strain on the capsule leading to stretching over time. An internal bra procedure with something like Strattice or Galaflex mesh is used for correction of this if it is severe enough.

I am sorry to hear about the problem you are experiencing. You demonstrate your concerns nicely. Although some “falling to these sides” of breast implants is quite normal when you lie down, if this occurs “excessively”, it may be a concern to patients.  This phenomenon is called lateral displacement of the breast implant;  it may be of concern from the visual and the discomfort standpoints. Surgical correction is an option.
Generally, the lateral breast implant displacement can be corrected using an internal suture technique, decreasing the size of the pockets and moving the implants toward the midline.    In my practice, I use  a 2 layered suture technique (capsulorraphy) to close off the space laterally (towards the axilla). The use of acellular dermal matrix is an option (although not usually necessary)  especially if significant implant rippling/palpability is present.
I hope this helps. 

Implants rarely, if ever, fall out of the pocket. Rather, the pocket can be stretched over time. If the pocket is made too large in the first place, then the implant will be seen to be out of position or falling to the side immediately. In most cases, the pocket is made correctly, but may stretch over time so that the implant can fall to the side.
When a breast augmentation is performed, the body forms a thin layer of scar tissue around the implant, this is called the capsule. In most cases this capsule stays soft and a little pliable and the breasts remain soft. If it thickens and shrinks, then we gat what is called capsular contracture and the breasts will feel firm.
A soft capsule is desirable so that the breasts feel soft, but can become stretched over time due to a number of factors; the tissues supporting the implant not being strong enough, the implant being too large and heavy, and the forces of the muscle can make this worse when sub muscular augmentation is done. This can result in the implants falling too far to the side too much. Some falling to the side is normal when laying down, just like normal breasts. If it is excessive, it usually indicated that the there is some combination of the tissues not being strong enough to support the implant, and the implant being too heavy. Solutions may include changing to smaller implants, and tightening the pocket. Just tightening the pocket with stitches will usually not last as the pocket will likely just stretch again over time. It may be necessary to add some reinforcement, a type of internal bra, with one of the meshes tissue reinforcement materials, to provide the necessary support.