>I have a 50 row, five column grid box on a form that I need to have the third and forth columns receive numeric input from the user. My question is how can I make it so the forth column value will be either the same value or a lesser value than the value in the third column entered by the user? I am using VFP 3.0
>

You can directly compare the value of column3 in the valid event of the textbox in column4.

If This.Value <= Thisform.Grid1.Column3.TextBox1.Value
   ** do anything here
Else
   ** do another thing here  
Endif

Or

If you can directly compare the value of the table field in the controlsource you used:

If This.Value <= mytable.amount
   ** do anything here
Else
   ** do another thing here
Endif