题目：译电文。为使电文保密，常按一定规律将其转换成密码，比如按照以下规律：将字母A变成E，即变成其后的第四个字母，相应的最后四个字母W变成A，X变成B，Y变成C，Z变成D，小写字母也一样。现在代码如下，麻烦把当字母是最后4个时的代码填下，谢谢大家了！
Private Sub Command1_Click()
Dim string1, string2, c As String
string1 = Text1.Text
strl = Len(string1)
For i = 1 To strl
c = Mid(string1, i, 1)
If (c >= "a" And c <= "z") Or (c >= "A" And c <= "Z") Then
c = Chr(Asc(c) + 4)
If                              Then
'当字母是最后4个时
c =
End If
End If
string2 = string2 & c
Next
Text2.Text = string2
End Sub
代码如上，当字母是最后4个时，If后要怎么写，c=多少呢？大家帮个忙！

一个一个付值不可以吗？你上面那个IF范围不用包括最后四个字母的啊

一个个付值也行，不过我想用个范围来表示它们，还望哪位高手发段代码过来，谢谢！

Private Sub Command1_Click()
Dim string1, string2, c As String
string1 = Text1.Text
strl = Len(string1)
For i = 1 To strl
c = Mid(string1, i, 1)
If (c >= "a" And c <= "v") Or (c >= "A" And c <= "V") Then
c = Chr(Asc(c) + 4)
ElseIf (c >= "v" And c <= "z") Then
c = LCase(Chr(Asc(c) - 54))
ElseIf (c >= "V" And c <= "Z") Then
c = UCase(Chr(Asc(c) + 10))
End If
string2 = string2 & c
Next
Text2.Text = string2
End Sub

Dim string1, string2, c As String
string1 = Text1.Text
strl = Len(string1)
For i = 1 To strl
c = Mid(string1, i, 1)
If (c >= "a" And c <= "v") Or (c >= "A" And c <= "V") Then
c = Chr(Asc("c") + 4)
ElseIf (c >= "w" And c <= "z") Or (c >= "A" And c <= "Z") Then
c = Chr(Asc(c) - 22)
End If
string2 = string2 & c
Next
Text2.Text = string2