1996 Chevy Cavalier - Car sputtering when gas low 1/4 tank

Randomly shuts off? The only "shutdown" for a car is the security system. I do not believe you have a shutdown for low oil pressure. I have seen cars shut down due to the gas cap being loose. I think you have had bad oil, even if you changed it, it could have varnished the hydraulic lifters which will give you the sputtering. This may be remedied with an anti-gumming agent. I know that the Dodge Intrepid (not your car- read on) had a poor design where the engine would gum up. Dodge makes an additive specifically for the oil of this engine that removes the gunk. This is what I would try. Other backyard methods like using diesel fuel do work and I have used them, but I DO NOT recommend this for you. Over the counter stuff at the department store may work, but I know that the Dodge stuff worked very well.

clean the spark plugs

The filter should be in the fuel line under the car between the gas tank and the engine.
On some models it is in the engine compartment, and on others it is in front of the gas tank.
You squeeze a clip together on one end and the other end has a threaded pipe nut.
Make sure you release the pressure in the line before you take the line loose.

Tank sending unit is bad.

TAKE VEHICLE TO A REPAIR SHOP THEY HAVE TO DRAIN THE TANK, PROBABLY REPLACE PUMP DRY OUT LINE AND INJECTORS,AS WELL AS FILTER, CYLINDERS. PROBABLY GONNA HAVE TO REPLACE PLUGS.

Without seing it, this is impossible to answer. Get to know a good Mechanic.

replace sending unit in tank

Because you have limited gas, the small amount of gas that you have in there isnt being sent thru your fuel line because ur on a slant. Thus when your level, gas is distributed evenly in the tank and fed thru your fuel system. duh.

Might be something in your gas tank that gets stuck, I've heard of people putting whiffle balls in peoples gas tanks and that happens.

it is most likely **** in your gas tank  the **** gets stuck in the fuel pump and then it wont send fuel let me know what you  think