@nooraalhamade , I'm not a doctor or medical professional, so it's not totally clear to me what "fourth-degree tumor" means. (And my main experience is as a pancreatic cancer patient.)

Did they completely remove a tumor on the prostate, or completely remove the prostate itself? If the latter, where exactly is the fourth-degree tumor, and do they have any evidence of metastasis?

(Stage IV is the generic reference for cancer that has metastasized to locations distant from the original tumor, and from what I can Google, it looks like individual prostate tumors are scored on a Group Grade and a Gleason Score: https://civatechoncology.com/ )

I ask because numerous metastases often drive surgeons to declare the cancer "inoperable, and too many mets can be too many targets for radiation therapy, leaving a "systemic" therapy (delivered throughout the bloodstream) as the only practical option.

While "cytotoxic" (chemicals that kill fast-growing cells) are what people think of in general, that is the older, traditional "chemotherapy" administered by IV, chest port, oral tablets, etc. Other therapies with the similar delivery routes include "immunotherapy" and "targeted therapy." The first paragraph of this article gives an overview of the differences: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3967236/
The key point is that some of these options are much less toxic and much better tolerated than traditional, cytotoxic chemotherapy.

Depending on the type, locations, and spread of the cancer, there are surgical options like HIPEC and HITHOC that attempt to remove all tumors that can be found, followed by "washing" the area directly with cytotoxic chemicals. This applies the chemo drug directly to areas where obvious tumors were removed and aims to kill any malignant cells that were missed by direct contact. It's an aggressive surgery (CytoReductive Surgery) with significant recovery required, but it avoids the generalized toxicity of injecting chemo directly into the blood stream.

Radiation is always a consideration, and may be possible if there aren't too many locations to hit and if sensitive organs are not involved.

There is a category of brachytherapy or internal radiotherapy where radioactive pellets / seeds can be implanted directly into a tumor, providing better targeting and less radiation exposure to other organs. I've heard of this being used before on prostate cancer, and stumbled on a new variant of this called the CivaSheet and CivaString: https://civatechoncology.com/ as long as there aren't too many tumors to attack.

There are a lot of options, and a lot of doctors might not be aware of the latest. If your oncology team doesn't offer any better options, please seek a second opinion from a major center with expertise in this type of cancer. And remember that even the major centers might only be aware of (or refer you to) clinical trials at their own center. Be as proactive as you can about seeking out clinical trials at other locations. Organizations like CancerCommons might be able to help. Check out https://cancercommons.org/news/prostate-cancer/