I don't know what the rule is on this, but we put our 4" cans with the edge closest to the cabinet over the edge of the counter. That way, they shine right in front of you when you are standing at the counter. A 6" would get pushed back farther if you use the edge. Stand at a counter where you would work comfortably and look up. My thought is that you want the center of the light just in front of you and hitting the counter in front of the upper cabinets. To close and they wash the cabinet more than light your work space and too far back, the center of the light will hit your head rather than toe work area. Make sense?

What about rotating the island pendant lights 90-degrees? I'm ok with the 3" cans over the sink. Or even 1x 4" can. The under-cabinet lights are great for coutertops, because with cans, regardless of their location or size, will create shadows due to your body being in the way. The rest of the lighting I'd change. First, are the 4" cans 120-Volt, or Low-voltage cans? Low-voltage cans won't give you the light you're looking for. But either way, I'd make them 5". I'd pull the can by the oven back so it's centered on the doorway(it'll cast plenty of light for the oven), and I'd also put 3 more in a straight line(walking from the oven towards to the range), evenly spaced. Like "brooksms" said, centered in the walkway(which is about center of the aforementioned doorway). I'd also put 2-3 cans on each side of the island(between the island & range, and between the island & fridge area), again, centered in the walkway, evenly spaced. And eliminate the 4 cans you have around the island. Yes, my layout added 1-3 cans, but it lights the whole kitchen more evenly, while still serving the purpose you want. Also, the 4 cans you have around the island, being that close to the pendants, will create shadows over the island. And the pendant lights(depending on the size and type of bulb) might not be able to compensate for this lost light created by the shadows. I'm a 9-year journeyman electrician, and I've been wiring custom homes for that entire time, just don't want you think I'm pulling these ideas out of my rear. :o) And like I tell people all the time, better,(and CHEAPER) to put more lights in now, than it is to add them later. You can always put the cans on a dimmer to lower the light if it's too bright.

I didn't go by any rule or any particularly pointed advice, I just did what I wanted. I did 4" cans at approxamately 23" off the wall. I spaced them out about 4 feet or so, but really I lit areas, not so much spacing. I put 1 over the stove, one to the left, one to the right, to the left and right of the sink, in front of the fridge (further out) I lit my sink area with a hanging fixture and pendants on the island. I added 1 recessed over the island, 1 in the "entryway" and one in the center to even the lighting out. I bet I will have a bit too much if I turn the pendants and recessed on a full blast, but I was planning to use them somewhat independantly and both on dimmers.

I'd prefer the closer-to-the-wall option - but also make sure that the light from a single can over the sink overlaps with light from adjacent can lights. Otherwise you'll still experience shadows while working. My sink is illuminated by undercabinet LED strip and three adjacent can lights. The cans are about 3' apart (one is centered above the sink cabinet and the others are equidistant on either side) so there are no shadows. I have upper cabinets on that wall and the cans are ~20" from the wall. The undercabinet LED strip is about 10" from the wall. This might seem overkill, but with all the lights on, my sink is a bright workspace and I love being able to see everything well.