Try this
Sub Demo()
    Application.Calculation = xlCalculationAutomatic 'set calculation mode to automatic
    Dim ws As Worksheet
    Set ws = ThisWorkbook.Sheets("Sheet2")  'change Sheet2 to your data sheet
    'check condition for B4=FALSE
    If Not ws.Range("B4") Then ws.Range("D3").ClearContents
    Application.Calculation = xlCalculationManual   'set calculation mode to manual
End Sub
NOTE : Use of SELECT should be avoided. See this for details.
EDIT : As @Slai pointed out in the comment below, If Not ws.Range("B4") will return TRUE if text of Range("B4")is either False, 0 or TRUE and FLASE are entered in Cell B4 else add another IF condition in code to check whether the entered value is BOOLEAN or not. Thus update above code as
Sub Demo()
    Application.Calculation = xlCalculationAutomatic 'set calculation mode to automatic
    Dim ws As Worksheet
    Set ws = ThisWorkbook.Sheets("Sheet2")  'change Sheet2 to your data sheet
    If VarType(ws.Range("B4")) = vbBoolean Then 'check whether the cell value is boolean
        'check condition for B4=FALSE
        If Not ws.Range("B4") Then ws.Range("D3").ClearContents
    End If
    Application.Calculation = xlCalculationManual   'set calculation mode to manual
End Sub