2 gallons is 9 litres uk, its hard to say water schedule without actually being there as some plants i water every day or 2 and others once a week, and im normal soil,as a rule of thumb if you can push your finger up to the knuckle without moisture they are too dry, in the pics they look dry,and 9 liters for 4 plants a week is not alot in 80 degrees, try watering more often, you dont say how old they are, if young you may want to lower/stop nutes for a week or 2 as this could be burning(browning in pic) but try watering first,keep them moist, let them dry out but only untill when finger poked in u can just feel moistuire on tip at about2-3inchs down, as i said, i had 10 of these, only 2 have gone, both slow, both washed out, both slightly out of shape,and i know my set up is ok as 46 others in same room on same diet fine!