If you need a weak reference to the cell value:
object field = d.Rows[0][3]
or
object field = d.Rows[0].ItemArray[3]
Should do it
If you need a strongly typed reference (string in your case) you can use the DataRowExtensions.Field extension method:
string field = d.Rows[0].Field<string>(3);
(make sure System.Data is in listed in the namespaces in this case)
Indexes are 0 based so we first access the first row (0) and then the 4th column in this row (3)
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