I would personally buy the first one you list - the dark grey charcoal - first. For your 4 suit rotation it will give you a contrast between the lighter and darker greys. I also have one of each color, and may consider that faux grey as a purchase down the road though.
Do you need to wear a suit on Friday? If so, you may want to go ahead and consider adding both of these. I don't work in politics so I can't comment on the suits you'll see other than what I see on TV, which mostly appear to be solid navy and charcoal. Do you think you could wear a plaid? Most of SS's plaids are in your face but maybe this suit could work. Or you could also get a grey pinstripe.
If you want to only have 4 for now, don't forget shoes and shirts as ways to change up the look. For example if you needed to wear the navy solid twice, you could on Monday or Tuesday wear a white shirt and dark brown shoes, and on Friday wear the same suit with a blue striped shirt and light brown shoes. Same goes for the light grey - wear black shoes one day and brown the other. If you have a little contrast very few people will even remember that you had on the same suit.