how to avoid infinite loops ine excel vba ?

[meen2525]

Hiii alll,
I have a requirement where I should check all the USED cells of an entire column and find if the first 4 letters is PT50.If its not PT50 it must be colored.However my code is going in infinte loop its processing all rows in the sheet.(those without data too).doesn stop.
HELPPPPPPPPPP how to avoid this infinite loop
this is my code
.Pleeeeeeeeeeeeeese correct it .seekin yr guidance .

Sub IbanNo()

Dim Result As String
LR = ActiveSheet.UsedRange.Rows.Count


For i = 4 To LR


val1 = Cells(i, 16).Value
Result = Left(val1, 4)

If Not Result Like "PT50" Then
Cells(i, 16).Select
    With Selection.Font
        .Color = -16776961
        .TintAndShade = 0
    End With

End If

Next i

End Sub