Hi Greg,
There is nothing wrong with using 4" cans for the look. The only real difference between the different size cans is the look and the spread of light. Because 6" cans have a wider beam you can space them further apart. With 4" cans you will want to keep them about 4 feet apart maximum and a little less would be better, I try to use 40-42" as spacing for 4" cans.
A 50 watt low voltage halogen bulb will give you much more light than a 50 watt incandescent and it is a more intense white light. In my opinion when you go to 4" cans, low voltage is a good choice. Overall in your situation with the 7'-6" ceilings I think low voltage 4" cans would be a good choice. Just be sure the bulbs are wide beam or floods.
Hope that helps. If you have anymore questions, please feel free to ask.
Paul
Light is not so much something that reveals, as it is itself the revelation.
James Turrell