Combo box and option buttons

you don't want the change fill range to be connected to the combo box change routine. It should be hooked to each option button's Click routine. Here is a different form of your routine. The click event for each of the option buttons calls the reset routine.

Sub OptionButton1_Click()
Call ajustFillRange
End Sub

Sub OptionButton2_Click()
Call ajustFillRange
End Sub

Sub OptionButton3_Click()
Call ajustFillRange
End Sub

Sub OptionButton4_Click()
Call ajustFillRange
End Sub

Sub OptionButton5_Click()
Call ajustFillRange

End SubSub ajustFillRange()
Select Case True
    Case OptionButton1.Value
        Sheet1.ComboBox4.ListFillRange = "T5:T278"
    Case OptionButton2.Value
        Sheet1.ComboBox4.ListFillRange = "T279:T552"
    Case OptionButton3.Value
        Sheet1.ComboBox4.ListFillRange = "T1150:T1639"
    Case OptionButton4.Value
        Sheet1.ComboBox4.ListFillRange = "T1640:T1676"
    Case OptionButton5.Value
        Sheet1.ComboBox4.ListFillRange = "T553:T1149"
End Select
End Sub