One of my Breasts is Loose and Sliding Around in the Pocket- What Can I Do?

Without photos it is difficult to provide guidance. Based on your history, you will need a revision to correct the asymmetry and close down the pocket . Go back to your surgeon and discuss this or seek a second opinion if you are not comfortable with the original surgeon. Good luck.

I would strongly recommend converting to a subfascial plane to create lasting support for a revised and higher position

By the fourth month after a breast augmentation, everything should be healed. I would consult with your plastic surgeon and have them examine you to see what can be done. A small revision surgery may be needed to correct this.

Breast implants, whether above or under the muscle, are held in place long-term by the scar capsule that forms. If it is out of position after 4 months, then it will likely need to be revised. Most of the time this can be accomplished with a procedure called a capsulorrhaphy. Of course without personally seeing you it is not possible to make a definitive recommendation, but it sounds like something that can be fixed without a significant recovery time.

It's difficult to tell entirely from the photo you submitted exactly what's going on.  It looks to me like your implant position is different on each side and that you'd definitely benefit from a breast lift.  Trying to correct your asymmetry only with implants may be difficult.