And for anyone who doubts the other 3 'know'? lolz they know! And I dare-say they approve, eh? :->
Eh, it’s always theoretically possible, but l’ll need to see a lot more evidence than we have so far. The dynamic in this arc is completely consistent with groups of friends where one, or more than one, pair is extremely close—if two besties are suddenly on the outs it throws the whole group off.
These girls still strike me as utterly unable to stifle their squees if they knew the truth. Maybe Rei-chin, who seems a little more mature and laid-back, but really I just don’t see it.
I'm still calling that the other 3 will have discussed it without them at some point, a while ago, and had a few discussions, and have varying ideas of it. They're gonna tell their friends they're dating and one of the friends is gonna be like "HAH! I knew it!" and it'll be revealed they've been discussing the possibility for a while.
That's certainly one plausible possibility.
As has escaped the notice of exactly zero of its readers, the central premise of this series is "Nanoha and Chidori are dating, but it's a secret." There's almost no chance at all that this particular author would ever go for a "Hey, you know what? Everyone knew all along!" gambit for the premise-changing reveal.
Judging by the handling of the Igarashi arc, my sense is that at an absolute minimum we would have gotten some clear hints in the form of raised eyebrows, quizzical glances, etc. as clues that the girls know what's going on with the Duo of Adorableness.
Somebody in the group suspects and is keeping quiet about it--certainly. But everyone? I really think not.