Do they have any access to grazing? If so, is the pasture good quality and not over-grazed?
9 flakes of hay per day doesn't sound like a lot between 4 horses, but if they're getting lots of grazing then it shouldn't be an issue. You usually want to aim for 1.5-2% of the horse's (ideal) body weight each day in forage, whether that is pasture grazing or hay. Your TB's may need a higher percentage than this.
I like that you're feeding a ration balancer, which I prefer over grain in almost all cases (although I'm not familiar with the particular one you're using and they don't list their ingredients in that link). I assume you mean you're feeding 0.4 kg (not lb) per feeding, for a total of 0.8 kg per day, which would be the recommended amount.
I'm assuming Abbey and Charlie are hard keepers, given that they're both getting Shurgain and beet pulp (and that they're TB's, who are stereotypically hard keepers). I'm not sure they're getting proper nutrition from the Shurgain/Trimax. Both of those have very short guaranteed analysis lists, which means that there's a lot of stuff that they've probably left out. I'd consider swapping them to the same ration balancer you're using for Nishka and Aurora and supplementing extra calories with (more) beet pulp, rice bran, and/or alfalfa (pellets, cubes, or hay).
You can try an online resource like FeedXl.com to really get a good look at their nutritional profile, but it's a little pricey when you have 4 horses ($30/month or $90/year). I started using it very recently for my horse, discovered that he was not getting enough copper (which is necessary for hoof health- his hooves were growing VERY slowly), started changing his diet, and within a week his hooves were already growing noticeably faster! They don't have any of your feeds in their system already, but say that they will add feeds on request. I'd recommend asking them to add those before you sign up if you want to go that route