Ya we have a all in one hp wifi unit. I think its a 8600 series one. We had a 8400 series one and i went to costco to buy all the inks it needed one day. It also needed a printhead but we were dealing with that. So the cost of the inks was approaching $85 or so I think, and cheap knockoff printheads were like $25? So I'm thinking for $100 I could get this one back up to snuff...when I see the 8600 series one on sale for I think $165? So I just bought a new one lol. Bonus is that I can print right from my phone without any special app or anything.
For instance though, the HP 8630 is going for $199.99 at office depot. The 8610 is going for $89.99.
If you get the combo pack for your c4640 that comes with a 60XL (so the larger black ink) and the normal sized color cartridge it is $75.49.
But, the reality is the 4640 has a better optical scanner I believe, and no auto document feeder to make it larger, so for compactness and movability, the one you have is likely a better choice. They do also make newer ones like yours, but I didn't have time to compare resolutions.
I'm out of the picture scanner game anymore. Todays cameras are so good, at least for me, I don't have any reason to scan in pics to touch them up hardly ever.