I have a slight double bubble what can I do to ensure it doesn't get worse? (Photos)

Thank you for your question.
At 4-5 months the pockets around the implants are starting to stabilize and hopefully the implants will not drop further.  A double bubble appearance has several possible causes.  
First is the implant dropping too low relative to the breast tissue and the natural IMF.  In your case the current IMF position is just below the incision.  If the incision placement was at the desired IMF as planned by your surgeon, then there has been some downwards shift of the implant over time. If your incision was planned above the IMF where it currently sits, then the implants are likely in a stable position overall.
Second is a constricted base to your natural breast tissue and in particular the lower pole.  Some patients are very short from the nipple to the original IMF (4-5 cm) and larger implants that are wider than the base of the natural breast tissue can lead to a transition from the curve of the lower pole breast tissue to the curve of the (wider) implant.  This happens more commonly when your natural breast tissue is very dense and will not spread out softly over the implant or when an implant is chosen that exceeds the base diameter of the breast (too wide or big an implant).  The firm breast tissue in effect sits like a hockey puck on the larger breast implant (sorry for not having a better description for that).  
Surgeons sometimes approach this with shaped textured implants to force more expansion of the lower pole, or "scoring" the lower pole breast tissue to weaken it so that it is more likely to stretch.  A natural result for a 425 cc implant might need 8-12 cm of final distance from the nipple to the fold depending on how large your breasts were to start with.  You can recruit 1-2 cm from the skin below the fold during the surgery but ultimately the rest has to "stretch" as the implants settle to reach the ideal final length.   Controlling the degree of stretch can be difficult as it depends on your natural healing and scar formation (genetics), activity level, use of supportive garments as well as the degree of release or support the surgeon creates during the surgery.
The third cause is dynamic and relates to the natural breast tissue sliding up and lateral when you lift your arms or contract the pectorals muscle.  The breast is attached to the muscle (assuming the implants are below the muscle) and when the fibres of the muscle are pulled up or contracting, the breast slides with them.  That may be a component of what you are seeing when you lift your arms as well.
In the end it is best to see your original surgeon and discuss your concerns.  Regular checkups in the first few months allow us to work with you to optimize activity level and bra use to try to get the final shape as close to ideal as possible.

Thank you very much for your question
and photographs. From your pictures, it does appear that you are beginning to
see a slight ‘double bubble’. This is in reference to the ability to see the
bottom curve and fullness of the implant, below the curvature of the breast. Unfortunately,
there is no way to prevent the ‘double bubble’ from progressing, however it is
possible that the situation will not get any worse. I recommend that you make
an appointment with your operating surgeon right away, as it is important to
have this situation assessed in-person. Your board-certified surgeon can make
an accurate diagnosis and provide the best recommendation moving forward, which
at some point, may include a breast augmentation revision.

Thank you for your question.
You do have a slight "double bubble". This may not get worse. Massage may or may not be helpful. Please discuss this with your operative surgeon. Please follow his advise and keep an eye on it.  All The Best !