After digesting what we all wrote here we should consider red elves charging green bowmen 1 from the side and obviously touching with its side to the front of blue bowmen 2.
How to position remaining two stands according to the rules is really the key question here 
Wouldn't be an issue as the charge will be to the front unless the line of sight is blocked by another unit.
Please consider I would like to charge Unit Bowmnen 1 (green) from the original picture.
1. According to the rulebook's "x/y" charging rules, it is obviously flank charge.
2. If you play "45 degree - zones" then imagine puting them 3cm to the right side of the picture to avoid uncertainity that it is actually the flank charge.
