Can I get the All-on-4 implants only on top?

While this CAN be done, we can't ALWAYS do so.  If the lower arch is missing several teeth, it can affect the bite forces on the new upper restoration, leading to failure of the all on four.  In SOME situations, it works out just fine, but not all.

You can certainly get an implant supported denture on your upper arch only. If you only have a few missing teeth in your upper jaw, they can also be individually substituted with single implants. However, a comprehensive exam with tomography will allow your dentist to make the recommendation that is best for your case specifically. The most common reason for missing teeth is tooth decay or advanced gum disease. These issues tend to spread in the mouth, affecting surrounding teeth. More often than not, if you have missing teeth because of gum disease, the future of teeth surrounding them is likely short. Another factor to consider is your bite. Once you place an all-on-four denture on either arch, it will likely result in malocclusion against the jaw that remains incomplete or imperfect. This is why we typically recommend for both jaws to be optimized, even if one is done first, and the other, a short time after.

Maybe. Four implants usually work out best when opposing another four implants on the opposing arch. When there is natural dentition opposing the implants, then we consider six because of how natural teeth produce more load (force) than acrylic teeth. A comprehensive evaluation including a CT scan will have to be conducted. In cases where the sinuses are too extended forward, we may have no choice but to only be able to space four implants. Not only cases are the same, and realistic expectations have to be discussed on an individual basis.