I see two possibilities here 1) something is killing them 2) they are killing each other.
1) Predators: Can anything get to them? If rats or anything smallish (larger predators would likely take the chicks away and/or kill more than just a few) can get to them- you need to predator proof your set up. Cover/close holes ect. Going out in the middle of the night and turn on a flashlight may also tell you what is sneaking in. Rat sign (poops, foot prints in the dust ect) can often be found if you look, if that is the problem. 4 weeks old is pretty young for cannibalism, which is the other possibility.
2) Cannibalism: usually when there is overcrowding, boredom, or inappropriate diet (usually protein deficiency). Or a combination of them. 4 weeks old is young for cannibalism, but is not out of the question. The meat birds grow very fast, and have a high protein need. Standard medicated chick start is probably not sufficient for them. I would get them (the meaties) some broiler food. You will need to feed and keep the leghorns separate from the meaties. Are there lights on 24/7 in their brooder? If the lights are out at night, they should be sleeping- and not likely to eat each other. Space and boredom- how big is the area for 50 birds? You can tell if they are eating each other by watching them for a few hours. Keeping in mind that meat birds will often die on their own because of their growth rate can outpace their heart's ability, and a bunch of birds tramping around will smash a fallen comrade and peck at it. You should not see chicks pecking out each other's vents. If you see anyone actually doing this, remove/cull the perps- it is hard to untrain them once they start, and it is a *really* bad habit to have- and they can teach each other this bad habit.