I just posted somethig related here
In your case you could try the next code:
Code
If iSeriesNumber = 4 Then
.Format.Line.Weight = 0.25 'Sets thickness = 0.25 to both markers-line and series-line
.Border.LineStyle = xlNone 'Hide the series-line
.MarkerStyle = xlMarkerStyleTriangle
.MarkerSize = 8
.MarkerForegroundColor = RGB(255, 255, 255) 'Sets white color to markers-line
End If