4" see Progress Lighting

4" have half the lumen output of a 5" or 6", so if you mean to ask ; what looks best AND is functional, then anything but 4" would be the answer. You need about 300 lumens per linear foot of counter top that is NOT undercabinet strip lighting. That is a lot of 4" cans.

1) Lumen output depends on lamp type but there is still plenty of light (eg, a MR16 lamp typical for 4" trims is commonly 500-700 lumens)
2) To further offer information around the question- a 8' ceiling (or any ceiling) should have the least amount of recessed light as possible. Recessed lights are horrible for general lighting, especially in a 8' ceiling.
3) Consider supplementing the recessed lights with wall lighting or table lamps, even a ceiling mounted fixture.

1) MR16 bulbs would need to be spaced every 2' over kitchen counter tops. PAR75 lamps typical of a larger recessed are usually 1200- 1500 lumens.
2) Recessed lighting is used as TASK lighting over counter tops or ACCENT lighting elsewhere. It is rarely used as GENERAL lighting. However, between task and accent lighting, it is rarely necessary to add an additional grid of recessed lights in the middle of the room to accomplish adequate general lighting, UNLESS YOU USE 4" CANS WITH A LOWER LUMEN OUTPUT!
3) MR16 BULBS tend to have a strange glare compared to other bulbs and you should use as few of them as possible regardless of ceiling height or how many real recessed you use. Therefore, to use fewer cans, use the bigger ones that have a higher lumen output.
4) The prismatic effect of the MR16's multiple reflectors makes the resulting light look phony and off putting.
5) MR16 bulbs are way less reliable than PAR bulbs, and I have replaced plenty of them with standard 5" cans over the years.
6) 4" cans will be much harder to retrofit with LED lightening when it becomes necessary. They do not have enough room for the cooling fins to work properly,and your expensive LEDs will burn out very rapidly.