Assuming your data starts on row 2, think you're looking for something along the lines of this
Sub dboyd616()
Dim rng As Range 'the range to work with
Dim cel As Range 'individual ranges within rng
Dim lastrow As Long
'to stop screen flicker and speed things along
Application.ScreenUpdating = False
With ActiveSheet
lastrow = .Cells(Rows.Count, "A").End(xlUp).Row
Set rng = Range("A2:A" & lastrow)
'this For Each...Next is your loop
For Each cel In rng
If cel.Value = "TextA" Then
cel.Offset(0, 3).Value = "TextA"
ElseIf cel.Value = "TextB" Then
cel.Offset(0, 3).Value = "TextC"
ElseIf cel.Value = "" Then
With cel
.Value = "Null"
.Font.Bold = True
.Font.Color = vbRed
End With
End If
Next cel
End With
'turn screenupdating back on
Application.ScreenUpdating = True
End Sub