I'll say that based on my previous experience, your cherimoyas are not getting too much light from the pictures you have shared showing how far they are from the light. Low humidity, excessive heat, salt burn from over-fertilization could all be causes... But in my opinion too much light from a 400W CMH bulb more than 2 feet above the plants isn't the cause. I've grown cherimoyas under 400W CMH bulbs before (at 50% humidity) and the top of the plant was 8" from the bulb before it showed any sign of light/heat stress.
If it is less than 85 degrees at leaf-level, it isn't heat stress. I'm guessing it is low humidity, or less likely salt-burn from the water, soil or fertilizer that is making them look burned.
I should add that there is a very small chance that your CMH bulb is defective (with a broken / compromised outer jacket) and letting way too much UV light through, but when that happens it generally makes the bulb explode catastrophically (I'm guessing you would have noticed that), and even if it hadn't, all of your plants would be getting burned by excess UV light, not just the cherimoyas. If you notice yourself getting a sunburn from exposure to the light you'll know for sure-- but again, if the outer envelope fails the whole bulb usually explodes.
I'd suggest cutting back on ventilation and possibly adding a humidifier inside the tent. If temperatures inside the tent at leaf-level are 85 degrees or less you're fine with ventilation. Air movement within the tent (stirring air around with an oscillating fan) is more important if temperatures aren't too high, and they likely aren't in your tent setup with only a 400W bulb. If that doesn't help, I'd consider getting your water tested to see what the salt / dissolved solids contents are; it is possible that you just need a water filter to keep your cherimoyas happy.
Kevin