Is basement finished living area? I think this depends on basement air quality and what you hope to accomplish by adding a cold air returning the basement.

IMO

The basement is not a finished living area and the air quality is good. The reason for doing this would be to get the cool air from the basement up into the top levels of the house where the air is much warmer. With a four level side split there are three sets of stairs connecting the fours levels and there is no door at the top or bottom of the basement stairs. It is a very hard design to heat or cool evenly.

" It is a very hard design to heat or cool evenly."

That is undouptedly true and all the more reason not to mess around with diy, roll your own and home made desighn changes to hvac system. On the surface your idea sound's like there is a free lunch to be had pulling cool air from basement. It's far more complicated than sticking a return in the basement. For every cubic foot of air sucked from basement,another cubic foot must replace it. That make up air can only come from outdoors (a terrible idea) or withing the building(which is exactly how every room in building is usually desighned). Trade schools spend hours teaching techs this so just believe me when I tell you for every cubic foot of air "falling down stairs"another cubic foot already "falls right back up them". So if you want to heat and cool basement,you should have ducts in it. If you don't care to have basement heated and cooled,make certain there are no leaks in existing ducts ( both supply and return ). Now so you don't go away empty handed,a simple test that might tell you if theres a leak in supply or return in basement. Hang a bed sheet,thin plastic or similar light fabric to cover doorway leading to basement. If sheet moves toward basement there's a leak in return. If sheet moves away from basement there's a leak in supply. If sheet doesn't move at all the bad news is there's a "REMOTE" chance leaks of exact same size exist's in both supply and return. I'm assuming your air handeler is in basement or main trunks run through it,if not,little I've said is applicial for your house.

klem: A four level side split (1979) means that our lowest level is almost completely below ground but is just half the foot print of the house. We have a bathroom and foyer along with a work room and laundry room in the basement. Bathroom, laundry room, & work room all have ducts in them with registers. But all the levels are open to each other and no level has a door that closes it off - very open concept. Now based on what you you say about every cubic foot of air sucked from the basement, another must replace it, maybe we should be putting in a cold air return in the basement. We have one cold air return on each of the first 3 levels, but none on the fourth. Below is a photo of the steps from 2nd to 3rd level and then 3rd to 4th (basement). As you can see everything is open.

"a photo of the steps from 2nd to 3rd level and then 3rd to 4th (basement). "

Now I understand,you definatly should get a return in the basement asap. What was throwing me off is that in the backwoods where I live people always refer to the bottom floor as level 1 then as you climb closer to the sun numbers increase in size to denote respective floors. Evidently where you live one could flat foot off the 4th floor and walk away unharmed. Complicating matters was starting off with no supplies in basement then later having 3 to appear. By all means add a return on level 4.

A properly designed HVAC air distribution system has returns in every room where you have supplies except kitchens, bathroom and other areas where you would pick up unwanted smells to circulate throughout the home. quad levels are some of the hardest homes to heat and cool. The designers don't care if the owners are comfortable in the home and, properly zoning the levels is impossible do to lack room to install a correct duct system.

Adding a return to the basement would be a good idea. I did this in my house a few years ago in the finished part along with a supply. The return benefits both the basement and the rest of the house.

Thanks for all your contributions. I have to agree with HVAC, quad levels are hard to heat and cool. We will be adding in a cold air return in the lowest level. Hope it helps!

On what floor is the air conditioner? From your original text, it sounds as if it's upstairs somewhere. Where is it getting its air supply?

Hi mtv: The air condition is an outdoor unit connected to the furnace. We haven't had it on much since installing the grate on the cold air return duct in the basement but I'm sure we will need it on again soon.