You could add this function then use it in your query:
Public Function minDate(d1 As Date, d2 As Date, d3 As Date, d4 As Date) As Date
minDate = d1
If d2 < minDate Then minDate = d2
If d3 < minDate Then minDate = d3
If d4 < minDate Then minDate = d4
End Function
But I agree...