I went to Tweeter and auditioned the 6 & 4...couldn't actually tell too much of a difference.
I mainly (over 90% of the time) use my HT setup for movies and not music.
Here is the rest of my setup:
Receiver: Denon AVR-1706
Center: Polk Audio CSi3
Surrounds: Polk Audio LS FX
Subwoofer: Polk Audio PSW303
Should I get the RTi4 or RTi6? And yes, I'd love to get 8's but can't afford it right now. I have other parts of my HT setup I have to put money towards.