4" space between breast implants when laying down, is this normal? (photos)

I would agree that your lateral pocket could be tightened a bit to encourage more medial displacement of the implants.  But from your upright view, I would actually leave well enough alone.  There is a saying in surgery that "The enemy of good is better".  And that saying holds true here.  I actually think you have a very nice result and wouldn't tough anything.  There is too little room for improvement and far too much room to screw things up.  I hope that helps!  You look great!

It is natural for breasts to fall to the sides when laying down. If the amount of movement is of concern, please discuss this with your surgeon. Your pockets could be tightened laterally to minimize this movement.

I am sorry to hear about your concerns after breast augmentation surgery. You demonstrate and describe 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 (and/or bottoming out) 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 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, and the attached link (dedicated to revisionary breast surgery concerns), helps.  Best wishes.