Four years is plenty of time to have a revision facelift. Make sure to consult with a surgeon who is experienced in revision facelift procedures so that you can have realistic expectations of what can be achieved. Good luck!.
Since it's been four years, enough time has passed for you to have another revision facelift. Make sure you consult with a revision facelift expert who can address your concerns and help achieve your desired results.
After four years there is no contraindication to a revision but it is surprising that one would be required. A well executed facelift should last decades and even a lifetime.
You are at 4 years so should be fine. There will always be some loosening of the lift after a few years. Often adding some volume at this point will help. If you have very heavy skin you may notice sagging more quickly.
If you are four years since your mid face procedure and you feel you have had a relapse, it is certainly reasonable to have a secondary procedure.
A picture would be awesome. There is no exact number of times. One can expect touch up surgeries as the face ages. The technique certain has an influence on how quickly facial surgery can relapse. Securing the lifted cheek to strong tissue and with slowly dissolving suture or permanent suture.
Most rejuvenation procedures have a slightly longer longevity that 3- 5 years. If you feel that your skin is relaxing, you may want to consider a less invasive procedure, such as C02 resurfacing where the skin envelope is tightened as opposed to another lift. Other devices such as RF will also tighten the skin, but I believe c02 provides the best results.