Yes, 4" is too small for 10 ft ceilings. 6" is better; I would go 8". Why not? Nice high ceilings and the crown will draw you eye up.

You don't say the style of your home. That always can impact trim.

You can always fake it - 4" moulding with chair rail cap or quarter-round mounted 2-3" below looks great when the gap is painted the same gloss white.

you can also do similar to modlife's suggestion but moving out over the ceiling, thats a look i really like.

Bigger is better - 8"