DB + tie + suit = fully acceptable in many places of the world, everything else is a matter of taste.
I don't usually wear DB's with suits, but I think it looks great when done right.
4 in hand works with any collar spread, if it needs to be bigger there is always the double 4IH.
This is also a matter of taste, but I have never seen an outfit with a Windsor knot that I don't think would have been improved by switching it for a 4IH. Not trying to preach here, it's just...