Assuming data starts in A2, a comma is the separator and no event code for the sheet,
on a copy of your workbook, paste this into a standard module and give it a try, alter the sheet name as required
Code:
Sub Testing_1()
Dim lr As Long, i As Integer, j As Integer, ray
Application.ScreenUpdating = False
With Sheets("Sheet1")
lr = .Range("A" & Rows.Count).End(xlUp).Row
For i = lr To 2 Step -1
ray = Split(.Cells(i, 4).Value, ",")
If UBound(ray) > 0 Then
.Range("A" & i).Resize(, 4).Copy
.Range("A" & i + 1).Resize(UBound(ray), 4).Insert xlShiftDown
For j = LBound(ray) To UBound(ray)
.Cells(i, 4).Offset(j).Value = Trim(ray(j))
Next j
End If
Next i
End With
Application.CutCopyMode = False
Application.ScreenUpdating = True
End Sub