It seems odd to me that you would describe the sadness as unusual - after four years that's enough to be devastated for most of us! Letting that out is necessary...
Having grown up surrounded by pets - dozens of guinea pigs, cats and dogs as a constant, birds, rabbits, chickens and so on - I have experienced a lot of death but I never stopped getting myself attached to them even as an adult.
But to be honest, the hardest loss for me was those cats I tried and failed to rescue. One starving kitty in particular I took to the vet to clean up and I spent four days feeding it at home, but my ignorance on how to do it right, how to get it to go toilet and such, caused the kitten to lose even more weight, ultimately having me sign a death contract and put the poor thing to sleep.
I was bawling my eyes out watching the vet do the procedure, and sat outside with the dead cat in a box crying about how shit I am at life lol. Haunts me to this day.
Anyway the point is, it seems clear that there was nothing you could have done in this case, you did everything right and gave your friend a happy life. That's worth a smile or two, for sure! Just don't mess it up next time like I did XD