Disabling comboboxes after a certain limit has been met - vba

Question

I have 4 comboboxes called:
cboOption1
cboOption2
cboOption3
cboOption4
Each of them can have one of 2 values, either 10 or 20 points, which the user selects from a drop down. However, the user can only select a maximum of 40 points across all drop downs, after which the remaining dropdowns that follow will be disabled.
E.g.1
cboOption1 = 20
cboOption2 = 20
Then
cboOption3.Enabled = False
cboOption3.Enabled = False
E.g.2
cboOption1 = 10
cboOption2 = 20
cboOption3 = 10
Then
cboOption4.Enabled = False
E.g.3
cboOption1 = 10
cboOption2 = 10
cboOption3 = 10
cboOption3 = 40
Then
None disabled
My problem
The difficulty I'm experiencing at the moment is figuring out how to flick the comboboxes on/off in response to user input. This is my code so far, it doesn't seem to be working too well, any idea how I can improve it.
Code so far
Sub checkTotal(var1, var2, var3, var4, total)
so1 = Int(var1)
so2 = Int(var2)
so3 = Int(var3)
so4 = Int(var4)
total = Nz(so1, 0) + Nz(so2, 0) + Nz(so3, 0) + Nz(so4, 0)
If total > 40 And (so1 = "20" Or so2 = "20" Or so3 = "20" Or so4 = "20") Then
cboOption3.Value = ""
cboOption3.Enabled = False
cboOption4.Value = ""
cboOption4.Enabled = False
End If
MsgBox total
End Sub
Private Sub cboOption1_Change()
Call checkTotal(cboOption1.Column(1), cboOption2.Column(1), cboOption3.Column(1), cboOption4.Column(1), so1RunningTotal)
End Sub
Private Sub cboOption2_Change()
Call checkTotal(cboOption1.Column(1), cboOption2.Column(1), cboOption3.Column(1), cboOption4.Column(1), so1RunningTotal)
End Sub
Private Sub cboOption3_Change()
Call checkTotal(cboOption1.Column(1), cboOption2.Column(1), cboOption3.Column(1), cboOption4.Column(1), so1RunningTotal)
End Sub
Private Sub cboOption4_Change()
Call checkTotal(cboOption1.Column(1), cboOption2.Column(1), cboOption3.Column(1), cboOption4.Column(1), so1RunningTotal)
End Sub

Answer 1

Please, read my comments to the question. I'd suggest to do that this way:
cboOption3.Enabled = CBool((cbo1Value + cbo2Value)<40)
cboOption4.Enabled = CBool((cbo1Value + cbo2Value + cbo3Value)<40)
Tip: use proper input parameters data type and remove total from input parameters.
Sub checkTotal(ByVal var1 As Integer, ByVal var2 As Integer, ByVal var3 As Integer, ByVal var4 As Integer)
See: Data Type Summary (Visual Basic)