Hello, thank you for your questions regarding Permalip lip augmentation. In regards to the difference between a 4 mm and 5 mm Permalip lip implant, there is basically little to no difference in terms of complications rate. By far the majority (95%) of my Beverly Hills and Las Vegas patients select the 5mm Permalip lip implant. They choose the 5mm for both the upper and the lower lip. My patients choose the largest Permalip implant size to get the biggest lips possible. in my experience, complication rates between the 4mm and the 5mm are the same. Complications from the Permalip permanent lip implants include scarring, implant shifting, lip asymmetry, lip implant infection, lip implant rejection, need for revision surgery, partial or complete lip numbness, and sensation of a foreign body in the lips. Fortunately, in experienced hands, these Permalip lip implant complication are very rare. Please go for the 5mm Permalip. I don't think you will be disappointed. If you have more questions regarding the Permalip permanent lip implant procedure, please call my office. Sincerely Dr. Katzen.