VBA - Write all possible combinations of 4 columns of data

Here's a generic approach which should work for any number of columns/values (within reason):

Sub ListCombinations()

Dim col As New Collection
Dim c As Range, sht As Worksheet, res
Dim i As Long, arr, numCols As Long

    Set sht = ActiveSheet
   'lists begin in A1, B1, C1, D1
    For Each c In sht.Range("A1:D1").Cells
        'col.Add Application.Transpose(sht.Range(c, c.End(xlDown)))
        col.Add Application.Transpose(sht.Range(c, sht.cells(Rows.Count, c.column).End(xlup))) 'safer...
        numCols = numCols + 1
    Next c
    
    res = Combine(col, "~~")
    
    For i = 0 To UBound(res)
        arr = Split(res(i), "~~")
        sht.Range("H1").Offset(i, 0).Resize(1, numCols) = arr
    Next i

End Sub


'create combinations from a collection of string arrays
Function Combine(col As Collection, SEP As String) As String()

    Dim rv() As String
    Dim pos() As Long, lengths() As Long, lbs() As Long, ubs() As Long
    Dim t As Long, i As Long, n As Long, ub As Long
    Dim numIn As Long, s As String, r As Long

    numIn = col.Count
    ReDim pos(1 To numIn)
    ReDim lbs(1 To numIn)
    ReDim ubs(1 To numIn)
    ReDim lengths(1 To numIn)
    t = 0
    For i = 1 To numIn  'calculate # of combinations, and cache bounds/lengths
        lbs(i) = LBound(col(i))
        ubs(i) = UBound(col(i))
        lengths(i) = (ubs(i) - lbs(i)) + 1
        pos(i) = lbs(i)
        t = IIf(t = 0, lengths(i), t * lengths(i))
    Next i
    ReDim rv(0 To t - 1) 'resize destination array

    For n = 0 To (t - 1)
        s = ""
        For i = 1 To numIn
            s = s & IIf(Len(s) > 0, SEP, "") & col(i)(pos(i)) 'build the string
        Next i
        rv(n) = s

        For i = numIn To 1 Step -1
            If pos(i) <> ubs(i) Then   'Not done all of this array yet...
                pos(i) = pos(i) + 1    'Increment array index
                For r = i + 1 To numIn 'Reset all the indexes
                    pos(r) = lbs(r)    '   of the later arrays
                Next r
                Exit For
            End If
        Next i
    Next n

    Combine = rv
End Function

For 5 Columns

Sub combinations()

    Dim c1() As Variant
    Dim c2() As Variant
    Dim c3() As Variant
    Dim c4() As Variant
    Dim c5() As Variant
    Dim out() As Variant
    Dim j As Long, k As Long, l As Long, m As Long, n As Long, o As Long


    Dim col1 As Range
    Dim col2 As Range
    Dim col3 As Range
    Dim col4 As Range
    Dim col5 As Range
    Dim out1 As Range


    Set col1 = Range("A1", Range("A1").End(xlDown))
    Set col2 = Range("B1", Range("B1").End(xlDown))
    Set col3 = Range("C1", Range("C1").End(xlDown))
    Set col4 = Range("D1", Range("D1").End(xlDown))
    Set col5 = Range("E1", Range("E1").End(xlDown))

    c1 = col1
    c2 = col2
    c3 = col3
    c4 = col4
    c5 = col5

    Set out1 = Range("G2", Range("K2").Offset(UBound(c1) * UBound(c2) * UBound(c3) * UBound(c4) * UBound(c5)))
    out = out1

    j = 1
    k = 1
    l = 1
    m = 1
    n = 1
    o = 1

    Do While j <= UBound(c1)
        Do While k <= UBound(c2)
            Do While l <= UBound(c3)
                Do While m <= UBound(c4)
                    Do While n <= UBound(c5) ' This now loops correctly
                        out(o, 1) = c1(j, 1)
                        out(o, 2) = c2(k, 1)
                        out(o, 3) = c3(l, 1)
                        out(o, 4) = c4(m, 1)
                        out(o, 5) = c5(n, 1)
                        o = o + 1
                        n = n + 1
                    Loop
                    n = 1
                    m = m + 1
                Loop
                m = 1
                l = l + 1
            Loop
            l = 1
            k = k + 1
        Loop
        k = 1
        j = j + 1
    Loop


    out1.Value = out
End Sub

For 4 Columns

Sub combinations()

    Dim c1() As Variant
    Dim c2() As Variant
    Dim c3() As Variant
    Dim c4() As Variant

    Dim out() As Variant
    Dim j As Long, k As Long, l As Long, m As Long, n As Long


    Dim col1 As Range
    Dim col2 As Range
    Dim col3 As Range
    Dim col4 As Range

    Dim out1 As Range


    Set col1 = Range("A1", Range("A1").End(xlDown))
    Set col2 = Range("B1", Range("B1").End(xlDown))
    Set col3 = Range("C1", Range("C1").End(xlDown))
    Set col4 = Range("D1", Range("D1").End(xlDown))

    c1 = col1
    c2 = col2
    c3 = col3
    c4 = col4

    Set out1 = Range("G2", Range("K2").Offset(UBound(c1) * UBound(c2) * UBound(c3) * UBound(c4)))
    out = out1

    j = 1
    k = 1
    l = 1
    m = 1
    n = 1


    Do While j <= UBound(c1)
        Do While k <= UBound(c2)
            Do While l <= UBound(c3)
                Do While m <= UBound(c4)
                    out(n, 1) = c1(j, 1)
                    out(n, 2) = c2(k, 1)
                    out(n, 3) = c3(l, 1)
                    out(n, 4) = c4(m, 1)
                    n = n + 1
                    m = m + 1
                Loop
                m = 1
                l = l + 1
            Loop
            l = 1
            k = k + 1
        Loop
        k = 1
        j = j + 1
    Loop


    out1.Value = out
End Sub