If you decide to switch to led lighting, 6" led modules are way cheaper than 4". 4" cans themselves are also somewhat pricy compared with 6" cans.

6". Much more choice and variety of bulbs available, and brighter ones (which you'll need with a high ceiling).

A 4" recessed can has a smaller aperture than a 6". This results in more light trapped within.

How about the Commercial Electric 4" modules from Home Depot, in brushed nickel ($40) or white ($35)? 8.3w, dimmable to 10%. Didn't look too bad when I saw them; they were quite bright. Lumens and color temp are on box (I think latter was 3000K).

If the cans are on the slope then you are stuck with the 6total inch because as far as I know they only make sloped housing in that size. If you want smaller LIGHTOLIER makes a regressed eyeball where the light actually adjusts inside the housing instead of stick out like typical eyeballs

"If you decide to switch to led lighting, 6" led modules are way cheaper than 4". 4" cans themselves are also somewhat pricy compared with 6" cans."

"6". Much more choice and variety of bulbs available, and brighter ones (which you'll need with a high ceiling)."

no criticism because that was excellent advice a few years ago and is a bit true still, but the fact is that the price and choice advantage of 6" retrofit vs 4" is narrowing and likely will be nil shortly. I would say especially if you think you may do a LED replacement not to worry about 4 vs 6.

"A 4" recessed can has a smaller aperture than a 6". This results in more light trapped within."

for incandescent, cfr and most par applications, yes , for led retrofit, no.

While I agree that led retrofit lamps do not suffer significantly from light being trapped within the lamp like standard bulb types in a can (the fixture is never 100% efficient), I would hesitate to use a 4" LED recessed lamp as the light output is less than a 6".
In addition, the advertised claim of "light output equivalent to 40 watt bulb" is reminiscent of extravagant claims that have been made (and still being made) for CFL and a number of LED products.

If engineered correctly, there is little reason why 4" LED recessed lights cannot surpass the 6" LED recessed lights.
To me, good 4" recessed lamps today continue to command a premium over 6" recessed lamps which themselves command a premium over other LED and other bulbs (- CFL, etc), which make them less practical ($$ wise).

The new Nora 5 inch can is a winner. it's a little pricey, but uses Cree parts, but is a very nice 5 inch light with different trim options. Nora also has a 4 inch LED with vaulted trim, but it's real pricy, but will work with your ceiling. It's really a nice light. Typically the better companies are underrating their LED's, not like the CFL's where they overate them. Stay as far away from commercial electric anything. Their lights are junk.