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?

If there are no uppers and just open shelving, I would center them over the counters to make sure the cans are not behind you thus casting shadows in your work area. I am currently in a rental right now while my home is under construction and the kitchen here is 12 x 14 (+/-). There are six cans, but they are not situated properly and cast horrible shadows in the work/prep areas (see pics). I think they should have installed a few more cans to light up a couple of the darker areas as well. But, the fact that you will be installing under counter lighting will help tremendously. I don’t know how much under-counter lighting counteracts shadows coming from the opposite direction though.

I recently faced this dilemma and ended up with most of my 4" cans centered on the edge of the countertop. In your case, I would put the cans on the window wall closer to the wall because you won't have undercabinet lights there. Also, I have a can directly over my sink and then one on either side (about 3' apart) and it is excellent work lighting. This a more aesthetic consideration, but I would consider whether you could do a combo of pendants over the peninsula and then flush- or semi-flush mount on the window wall, 12" off the wall. Or maybe all pendants if they're not too long.

4" are small, we did them in our bathrooms and wishing we would have done the 6" in there too, consider 6" recessed for the kitchen. Our ceilings are 8' and we have a large kitchen, this is ours... six recessed 6" can lights, 2 pendants over the island and one light over each window (pendants aren't installed yet but you can see where they'd go).