I'm confused as to the range of brightness values associated with Landsat 8 Bands. I know that for Landsat 7 and below it has always been 0 - 255 but some of the literature indicates that Landsat 8 values range from 0 - 4096 since it's 12 - Bit. Is this a correct?
Yes, the difference is correct.
The reason for the difference between the two is the increased bit-depth of Landsat 8 (16 bit), when compared to Landsat 7 (8 bit). By calibrating the data to top-of-atmosphere reflectance the difference between the images should be minimal.
I don't think so. Landsat 8 range values are from 0 to 65536 (8 bits: 2^8 = 256; 16 bits: 2^16 = 65536) But usually values are between 4-5K and 21-23K.
Landsat 8's OLI and TIRS sensors feature a 12-bit dynamic range which translates to 4096 potential grey levels. The products, however, are delivered as 16-bit images (scaled to 55,000 grey levels). See USGS' page on Landsat 8.