Gas and Oil Home Heating Furnaces - 4" versus 6 " cold air duct?

Hello, thanks again for all of your help.

I am framing in my lookout basement and am now focusing on what will be a 13 x 16 bedroom/office. Currently, there is 6 " duct run for hot/cold air that terminates on the ceiling in front of the window (standard). I want to run a cold air return for this room but am forced to use 4 " duct instead of 6 ". When the air is on and I have my vents open downstairs, I get minimal or average air flow out of the vents anyway, thus, a 4" cold air return duct tapped into my main cold air return should be sufficient, shouldn't it? The run from the main duct to the vent will be about 8 ft total with 2, 90 degree angles and a 6" vent/vent boot. (Had to use 6" because I couldn't fine 90 degree angle 4 or 5 inch vent boots).

Again, any input would be great. Thanks again, Pat

P.S. I did not want to just tap into the main cold air return duct, which I could, because I wanted to have the cold air return on the floor for proper circulation.

if i understand correctly. those 4 and 5 inch 90 degree boots are not a stock item at DIY stores, but available at hvac supply houses. why do you have to use 4 inch?

I have to use 4 inch because there is only about 4 1/2 - 5 inches of space between where the back of my frame will be and the end of the main cold air duct I am tapping into. I am attaching my frame to the first joist, which is 7-8 inches from the wall and only have minimal space to use. I could tap into the main line with a vent right in the middle of the room but that would put my cold air return for that bedroom/office at ceiling level. Same heigth as my warm air duct but on the opposite side of the room.

What is better? Having 4" or having my return air at ceiling level? Thanks again, Pat