Please don't let yourself think for a second that you've "let him down." I can understand that this must have been a painful decision, but the alternative was to leave him in that shelter. Where he might have had a couple more days...and been utterly miserable and confused the whole time, especially since he was someone's beloved pet for most of his life. You saved him from a horrible death, and gave him instead four wonderful days and an easy (for him), beautiful exit, with someone to hold him as he leaves this world.

Not to mention a great, big group of friends from all over the world who will be thinking of him and praying for him on his last day.

I don't know you well enough to hug you, but I'm sending ((((HUGS))) anyway, because I just can't help it.