A question about cb radio antennas?

I have a simple Uniden pro 510xl radio.....i have a simple 4 foot cb antenna (i dont know much about it, all i know is its not a loaded antenna) and i want to get the best range out of my four watts.....i live in a heavily forested area and am pretty level with everything else around me.....i dont want to install the antenna outside and i was wondering if i could install it in my attic.......the walls in my attic are pretty thin with no insulation on the ceiling......no wires or pipes either.....there is little between the antenna and the outside world (i have a non-metallic shingled roof (also pretty thin).....so if i installed the antenna in my attic, will it affect my performance compared to if i installed it outside at the same height?.....i know it will attenuate the signal slightly if i installed it in my attic but i dont have a good idea on how radio waves in the cb frequency range propagate.....so could i still get 6 miles or so with the attic-mounted antenna, or is outside the only way?