They come from the brown chest. You get the brown chest as a reward when you get enough experience points to level up. Several random items can be released from the chest. A level 1 chest holds 8 items and will disappear when exhausted. I release 7 items then set it aside, waiting for another brown chest. After I release 7 items from the second brown chest I merge them which will give you 12 or 14 more items to be released before the level 2 chest disappears. Unfortunately level 2 is the max and won't recharge that I know of. No use in keeping it around.

Yes, it will deliver up to 4 slices under two conditions:
1) An open space beside the toaster for the new slice to pop into.
2) Enough time since the toaster last produced bread. It can produce 1 slice every 10 minutes. If you wait 40 minutes it should produce 4 slices all at once.
I keep my toaster in a crowded area (no open spaces near it) so it won't be popping out bread when I don't need it.