Back when I owned the 4800, I had this same question and came upon someone who had emailed Toshiba regarding this. Toshiba's response is below. Hope this helps!
I RECIEVED AN E-MAIL RESPONSE FROM TOSHIBA REGARDING THE BLACK LEVEL EXPANSION. THEY CLAIM THAT IN THE PICTURE SETTINGS THE BRIGHTNESS CONTROL IS ACTUALLY THE BLACK LEVEL EXPANSION. (GOES FROM -7 TO +7) IN THE OTHER MODELS THAT HAVE THE EPM BUTTON THE BLACK LEVEL EXPANSION WOULD BE EQUAL TO -7 SETTING AND WITH IT OFF WOULD BE +7. THEY CLAIM THAT WITH THESE SETTINGS YOU HAVE BETTER CONTROL AND CAN SET THE BLACK LEVEL CUTOFF TO EXACTLY WHERE YOU WANT. HERES THE TOSHIBA E-MAIL RESPONSE:
Black Level Expansion is the ability of a DVD player to provide richer/deeper black reproduction, and a wider contrast range by modifying the black cutoff frequency. The NORMAL mode in our DVD players assigns 7.5ire as the black cutoff. (ire is a scale of brightness, from absolute black ---0ire---to absolute white --100ire) If a consumer chooses the "Expanded" mode, they are setting black cutoff at 0ire. This will give them maximum contrast, and best absolute black images. Because of bandwidth, most technicians recommend the lower (expanded) ire setting only when using component video interconnects between the DVD player and the TV. The ability of the connected monitor to deliver a wide dynamic range is also crucial to optimizing the expanded contrast afforded by the 0ire setting.
Models with "Custom Picture Settings" there is no separate menu setting for Video Black Level; hence, no "expanded mode"
To adjust video black level, the consumer must go into the CUSTOM PICTURE SETTINGS menu and adjust from there. Here they will find a BRIGHTNESS feature that offers a variable setting scale from -7 to +7. This provides more flexibility than the traditional NORMAL/EXPANDED mode on our main line DVD players as the variable nature of this setting actually permits the customer to choose the exact black cutoff level that best suits the performance of their monitor.