Tom is correct as are the others. Let me go a little further into why storing this particular calculated data is poor practice. What happens if you make a mistake and miss entering a month? Now you have a minimum of two records to change. If you don't notice it for a while, you may have a bunch of records that all need to change.
However, I will load the bullets into your gun. You would never use the enter key for this purpose. You would need code in the form's BeforeUpdate event so the populating of the "fifth" field wouldn't happen until the record gets saved. Two columns doesn't make any sense.
The table should be:
ReadingID (autonumber FK)
MeterID (FK to the meter table so you know what meter you are entering data for)
ReadingDT
CurrentReading
PrevReading
The user enters the meterID, the ReadingDT, and the CurrentReading. When you save the record, your code looks at the most recent reading for the meter and picks up the CurrentReading value from that row by opening a recordset based on a query that returns the Top 1 record and sorts descending by ReadingDT with a Where value for the entered MeterID and copies it to the PrevReading on the current record. You have to allow for no record to be found so the code will work for the first reading on a meter.
The gotcha's. Your code also must verify that there is no reading missing between the previous date and the current date. So, if there is a missing reading, it won't let you add the reading for the date you entered because if it does, the PrevReading value will be incorrect.