Please confirm the model number of the refrigerator?
Here's a list of all the Whirlpool 4YET2* models that I could find, from this supplier and also this supplier as I couldn't find anything regarding YET2AL* models.
The answer above has a check list that mentions checking the door seals. Sometimes a visual inspection may not indicate a problem that could be there. If you haven't already done so, place a sheet of paper between the freezer door and the freezer door jamb and with the door closed normally i.e. no pressure on it by you to keep it closed, try pulling it out. It will come out with a bit of an effort but not easily and it definitely shouldn't fall out by itself. Do this at various places around the door, top, bottom and both sides. Also check the door alignment to ensure that it is parallel to the cabinet in case it is a hinge problem
Is the compressor running continually trying to get the temp down? If the compressor does stop occasionally then the problem may be due to a faulty temp sensor or control board thinking that the temp is OK when it's not.
Is the evap unit in the freezer icy cold i.e. 0°F to the touch?
Is the condenser unit under the fridge near the compressor warm to the touch?
If the answers are yes, no and yes then there's a problem with the sealed system.
The sealed system comprises the compressor, evaporator unit, condenser unit, filter dryer and associated piping.
If the temp is not right and the compressor is running continually, then this could be due to either a faulty compressor, a lack of refrigerant (leak? - look for oily residue on pipes near condenser and compressor) a blockage in the system (check for ice build up on the pipes near the compressor/condenser) or in the filter dryer perhaps.
To check the sealed system depending on your location you may have to to get a licensed refrigerator repair service. This is due to the environmental regulations regarding the handling of refrigerant gases