There are numerous ways of going about this. Perhaps the simplest for the table you described is to create a four-row, one-column table, then split each of the 2nd-4th rows into however many cells you want. See Demo1. This will result in a table in which the cell widths on each row are equal. Alternatively, for a table with some cells spanning two-or-more columns, you could start with the four columns then merge the cells concerned. See Demo2.
Code:
Sub Demo1()
Dim wdTbl As Table, i As Long
With ActiveDocument
Set wdTbl = .Tables.Add(Range:=.Paragraphs.Last.Range, Numrows:=4, NumColumns:=1)
With wdTbl
For i = 2 To .Rows.Count
.Rows(i).Cells(1).Split Numrows:=1, NumColumns:=i
Next
End With
End With
End Sub
Sub Demo2()
Dim wdTbl As Table, i As Long, Rng As Range
With ActiveDocument
Set wdTbl = .Tables.Add(Range:=.Paragraphs.Last.Range, Numrows:=4, NumColumns:=4)
With wdTbl
For i = 2 To .Rows.Count
Set Rng = .Rows(i).Cells(i - 1).Range
With Rng
.MoveEnd Unit:=wdCell, Count:=1
.Cells.Merge
End With
Next
End With
End With
End Sub