using a macro in VBA:

my solution +>

Option Explicit

Const iRow_Convert_Start As Long = 2

Sub One_To_Multiple()

Dim Extract_of_Col_D_Array() As String
Dim iRow_Convert As Long
Dim iRow_Convert_End As Long
Dim iRow_Into As Long
Dim iRow_Into_Start As Long
Dim iRow_Into_End
Dim j As Long
Dim Ws As Worksheet

Set Ws = ActiveSheet

j = 1
Do While (Ws.Cells(j, 1) <> "")
j = j + 1
Loop

iRow_Convert_End = j - 1
iRow_Into = iRow_Convert_End + 2

Ws.Range("A" & iRow_Into) = "Into"
iRow_Into = iRow_Into + 1

For iRow_Convert = iRow_Convert_Start To iRow_Convert_End

Extract_of_Col_D_Array() = Split(Replace(Ws.Range("D" & iRow_Convert), " ", ""), ",")
For j = LBound(Extract_of_Col_D_Array()) To UBound(Extract_of_Col_D_Array())
Ws.Range("A" & iRow_Into) = Ws.Range("A" & iRow_Convert)
Ws.Range("B" & iRow_Into) = Ws.Range("B" & iRow_Convert)
Ws.Range("C" & iRow_Into) = Ws.Range("C" & iRow_Convert)
Ws.Range("D" & iRow_Into) = Extract_of_Col_D_Array(j)
iRow_Into = iRow_Into + 1
Next
Next
End Sub