2 Sheets: "Database" and "Client Codes"
On Sheet Database,
Cell B4 is where one would type a Code Number.
Cell C4 is where one would type the corresponding Client Name.
On Sheet Client Codes,
Column A2 and down are Code Numbers.
Column B2 and down are corresponding Client Names.

I am hoping to make a Worksheet_Change that looks at Sheet Database Cells B4 and C4 and uses a Index match function to lookup the corresponding Number/Name from Sheet Client Codes.

I started with a Sub button to simply see if I could make it work. The Index Match formula is returning a 1004 error. The next step would be to make it a Worksheet_Change private sub that makes things simple for someone entering data. 

Sub Auto_Fill()
Dim lastRow, CodeToClient As Long, ClientToCode As Long
lastRow = Worksheets("Client Codes").Cells(Rows.Count, 1).End(xlUp).Row

With Application.WorksheetFunction
CodeToClient = .Index(Sheets("Client Codes").Range("A2:A" & lastRow), _
                          .Match(Sheets("Database").Range("B4").Value, Sheets("Client Codes").Range("B2:B" & lastRow)))
ClientToCode = .Index(Sheets("Client Codes").Range("B2:B" & lastRow), _
                          .Match(Sheets("Database").Range("C4").Value, Sheets("Client Codes").Range("A2:A" & lastRow)))
End With

If Range("B4").Value <> "" Then
    If Range("C4").Value <> "" Then
        MsgBox "B4 <> blank, C4 <> blank, Exit Sub"
    Else
        MsgBox "B4 <> blank, C4 = blank," & vbNewLine _
            & "Do C4 = CodeToClient.Value" 'B4 value dictates C4
    End If
Else
    If Range("C4").Value <> "" Then
        MsgBox "B4 = blank, C4 <> blank," & vbNewLine _
        & "Do B4 = ClientToCode.Value" 'C4 value dictates B4
    Else
        MsgBox "B4 = blank, C4 = blank, Exit Sub"
    End If
End If

End Sub
Thanks for taking the time to look,
Red