Sub Ventilation()
Dim Lg%, Plage As Range, Cel As Range, ShRef As Worksheet, Sh As Worksheet, Ref As Range
Set Sh = Sheets("Base"): Set ShRef = Sheets("Synthèse"): Set Ref = ShRef.Range("A4")
Lg = Sh.Range("C" & Rows.Count).End(xlUp).Row
Set Plage = Sh.Range("C3:C" & Lg)
Select Case Len(Ref)
Case 0
MsgBox "Impossible de lancer la macro", vbCritical + vbOKOnly, "Valeur erronée"
Case 1
For Each Cel In Plage
If Left(Cel, 1) = Ref Then Cel.Offset(, 4) = ShRef.Range("B4")
Next Cel
Case Else
For Each Cel In Plage
If Cel = Ref Then Cel.Offset(, 4) = ShRef.Range("B4"): Exit For
Next Cel
End Select
End Sub