Do you mean that at a nice equivalent dose for you personally, oxy is 4x subjectively "stronger" as a drug than morphine, or that oxycodone is, in terms of potency, 4 times stronger than morphine?
I hope you mean the former, because oxy being 4x stronger than morphine? Nuh-uh. More like 1.5, maybe twice as strong - not three, definitely not four.
Morphine is 1.5-2x more potent than oxycodone, but the low BA makes orally administered oxycodone stronger. Nothing like 4x stronger though.
Anyway at equivalent doses I find morphine much stronger than oxycodone, simply because it's a lot more sedative.
And, of course, it depends on how much Oxy you take and how you do it. Morphine is simple and basic. I use methadone and it´s 20 mg orally is equivalent to 10 mg of morphine IV. That is a fact!
2-4x is a huge difference. I've also read 1.5 somewhere, but I think 2 is the more commonly quoted figure.
Eg.
http://www.ncbi.nlm.nih.gov/pubmed/6783935
Heroin does feel subjectively stronger, though, because of crossing the BBB faster (and possibly due to short-acting intermediary metabolites like 6-MAM).