Does it stop knocking if you go above 4500 rpm.
Have you changed the oil any metal in it. Do you have a magnet in the oil pan or drain plug. I would drain the oil and take the old filter and cut it in half and see if its filled with junk. Also see if the oil is filled with crap. You can strain it with a shop towel and see if any metal in the rag.
Are the plugs clean. All the same. I think you already said i will look back.
Did you put a full tank of gas in it. If you used just a few gallons go to the race track and buy 104 octane or something better and see if it runs right. I got a feeling its not going to change things. Either you have major oil on top of the pistons issue or a spun bearing making funny noise. A cam and detune might help if the cam is wiped but if it is the cam engine will be filled with scrap metal. I got a feeling exploratory surgery is in your future.
Yeah i cant see any reason it would start knocking like crazy just after it has been setting. A compression check might be a good idea before you take it apart. Seems like something is wrong. I think a cam swap and lower compression might fix the problem but i think you will find the problems when you take the pistons out and check over the heads. It is very possible that it spun a bearing or had some other oil related issue after being sitting for so long and starting up dry. But i cant say anything for sure with out crawling under the hood and hearing it first hand.