I have a Canon iPF6400, so not exactly what you're comparing to, but similar. Based on my experience, the Canon is a wonderful printer if used several times per week. It needs to print in order to stay well maintained. If you don't print and leave it on, it might be good enough, but I think it will be dumping a good bit of ink in the process. I honestly even cringe at a 12-14ml clean cycle if I miss not printing for a few days.
From what I read, the HP can be off for a long time and pick up exactly where it left off. Also keep in mind the heads for the Canon are very expensive compared to the HP. Also, if you do have to replace the head, I imagine the ink dumping for the 4000 will be quite drastic, as in losing hundreds of dollars in ink just to re-prime the new printhead.
I have no direct experience with printing on metallic, nor how the HP inks handle metallic or canvas. But strictly from a user experience point of view, a sitting printer will be a nightmare, other than the HP perhaps which seems to handle this well.