Replace multiple "If Then" with "For" loop

Can anyone please tell me how to use "for" loop for this?

Function AFIX(cn As String, prem1 As Double, income1 As Double, Optional prem2 As Variant, Optional income2 As Variant, Optional prem3 As Variant, Optional income3 As Variant, Optional prem4 As Variant, Optional income4 As Variant

If IsMissing(prem2) Then
    prem2 = ""
Else
    If ((prem2 <> 20000) And (prem2 <> 25000) And (prem2 <> 50000) And (prem2 <> 100000) And (prem2 <> 150000) And (prem2 <> 200000) And (prem2 <> 250000)) Then
       MsgBox "Error: Enter a valid premium"
       Exit Function
    End If
End If

If IsMissing(prem3) Then
    prem3 = ""
Else
    If ((prem3 <> 20000) And (prem3 <> 25000) And (prem3 <> 50000) And (prem3 <> 100000) And (prem3 <> 150000) And (prem3 <> 200000) And (prem3 <> 250000)) Then
       MsgBox "Error: Enter a valid premium"
       Exit Function
    End If
End If

If IsMissing(prem4) Then
    prem4 = ""
Else
    If ((prem4 <> 20000) And (prem4 <> 25000) And (prem4 <> 50000) And (prem4 <> 100000) And (prem4 <> 150000) And (prem4 <> 200000) And (prem4 <> 250000)) Then
       MsgBox "Error: Enter a valid premium"
       Exit Function
    End If
End If