This doesn't look much like a Guards' Coat, more like a quirky DB town-ish overcoat.
Here is the effect of the martingale as Michael describes it (and I very much agree with it):
And here is just the right amount of the shape you are looking for:
(sorry to take myself as an example, but that was close to hand)
If you still want to go with 4 buttons, you may place them in a square, with the top ones slightly above natural waist. You have the option to button up the top or let the lapel roll through to the lower button line. Here is an example that I liked so much that I had it made in gray flannel, just as it is in the picture:
(no martingale on this one)
In mid-brown Donegal, however, I think Michael's suggestion of an Ulster-inspired design is best.
On the subject of fitted overcoats - I have a couple and I hate them: they are very uncomfortable. There is friction everywhere and I feel stiff in them. A more loosely shaped overcoat looks and feels much nicer to me - it is more fluid, moves better with me and feels unrestrictive. I feel dressed, rather than bandaged.