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

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. 

You can get an All on 4 done for the upper jaw only providing the plane of the lower teeth is level. If it isn't some of your lower teeth would need to be adjusted to accommodate the upper prosthesis.Wish you all the best

Generally speaking yes it is possible however it really depends on several factors including what your upper arch can currently support.
I have seen some dentists use all on 4 dental implants for upper or lower for all patients regardless of their situations. This one size fits all approach is not a good idea. For most patients a minimum of 6 implants on the upper ( all on 6) and 5 implants on the lower arch are the minimum i would recommend for stable long-term results.

There are options for all on 4 treatment on the lower as well. It would be in your best interest to visit the doctor of your choosing and have all questions concerning this procedure address. The doctor that you feel most comfortable with should be the one that does your surgery.

In a very simplified-generalized answer, yes you can get a single arch all on 4.  For a complete answer you would need to have a full complete exam and ensure that you are a candidate for an all on 4 procedure.  A dentist would like to know if your opposing teeth are healthy enough to function against a hybrid prosthesis and if it is a possible treatment to place an all on 4 prosthesis.

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.

Yes you can , as long as you have enough bone to receive the implants, and the bone quality is good, the all on 4, protocol, is a proven protocol, and done with a certified doctor, this will be the best decision you will take, because it can be done within just 1 visit.