It's hard to say what's going on here without a little more inspection/description. For starters: How much do they weigh? What is the protein content of their feed? Are they acting healthy and are they fully feathered? Are they skinny and lethargic? How are you raising them (pasture pen/coop/indoors/free range/etc)? Etc, etc.
I always assume I'm going to loose 10% of my meat birds. I don't loose that many every time, but it's good to assume you're going to have a batch that goes wrong for one reason or another. So, that being said, you haven't lost an absurd amount of birds yet, but just enough to start being concerned.
I would definitely inspect the flock rigorously asap. Check their crops, check for lice, check for bruising on the keel bone, check for discharge/build-up around the nostrils. Monitor their water and feed - make sure everything is flowing properly throughout the day and they have plenty of access to water. Make sure 60-70% of the birds can all access the feed at the same time. Etc, etc.
If another one dies, then eviscerate and inspect the organs. Not a fun job at all, but it can be very revealing when it comes to mysterious die offs. I would venture a guess that the 4 chickens that you lost in the last 3 days all died from the same cause and that the earlier die off in the brooder was a separate issue. Wishing you the best of luck with this and a successful harvest.
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