Those numbers are pretty alarming. Now would be a good time to research the internet to learn about the effects of ammonia and nitrite on fish. Way too large a subject to describe in a forum. Best I can do is give you a few points to research.
First off I'm a bit skeptical on your ammonia numbers since it is so high and it doesn't sound like you have any fish. However other things, like nitrite, do kind of confirm high ammonia. So ammonia could actually be that high.
Here's a very
good table for seeing how toxic your ammonia level is in your pond. Enter your ammonia level, 8, into the "Enter Total Ammonia" field and click "Recalculate Table" button. For your water temp and pH you'll see your water is extremely toxic to fish. However, if you added dechlorinator it can turn the toxic ammonia into safe ammonia. Your test is for total ammonia, both safe and toxic. The table in the link is normally used to break total ammonia into safe and toxic parts. But the dechlorinator changes that and there's no way to tell safe and toxic levels without
specific test kits. So it's impossible to tell if ammonia is actually your problem, but that's where the smart money is.
In your original post you said one fish lived. Is it still alive? If yes I would question your test results. Is your test kit strips or drops of liquid? Strips can go bad.
You can contact your water supplier to try and confirm the 4.0 ppm ammonia level. That's really high. If the supplier says ammonia should be something like 0.4 ppm you know your test kit is bad or you're using it incorrectly or there's something serious bad with your water delivery system.
You can also take a water sample to a pet store and have them test it.
A test for nitrite would be useful in figuring out the ammonia issue and also whether that's toxic also. Nitrite is what you get after bacteria converts ammonia. Another kind of bacteria converts the nitrite into nitrate. Having such a high ammonia level makes me think there is also a high nitrite level which is also very toxic to fish. Or did you miss type "Nitrate" and it should have been "Nitrite"?
Assuming all fish are currently dead and the ammonia test of your source water is really 4 ppm I would look into serious bio filtering. Things like "Trickle Tower", "Bakki Shower Filter", "Moving Bed Filter". At the level of ammonia you're reporting and your desire to keep a lot of Koi (or any Koi) in such a small pond means you have to be serious about fish keeping or you can expect to continue killing fish. You have the exact same conditions that high end Koi ponds have, huge fish load. That requires serious learning, filters and testing. You must stay away from all the typical toy filters normally targeted to water gardens. To give you an idea of what I'm talking about a normal mass market toy filter will say something like "for ponds up to 250 gals". For your pond and desired fish load you would want a filter that said "for ponds up to 3,000 gal", or maybe more. But filters that say something like "for ponds up to X gals" are generally going to be very poor filters for your case. Filters have to be sized by fish load, not pond size.