I have four (4) quail feeders (Milo & Cracked Corn) set up about 150 yards apart. What else can I do to increase the quail population in this area, which covers only about 40 acres of our ranch?

Make sure your feeders have plenty of cover. Feeders are varmint and bird of prey magnets.

Pray for rain like we had last spring and summer, and while you're at it, ask for it to continue into the fall!

Hard to do much with only 40 acres. But it never hurts to feed them! Water and cover is a must also. Come on rain!!

Yeah, we need more cover. I have a lot of wooden pallets for makeshift shelters/cover, and we did get a lot of rain last year.

Under ideal conditions 40 acres will only support 1 or two coveys. In other words you may have property to train, but you need a lot more land for a huntable population.
Best thing is neighbors with a lot of land and good habitat. Birds on plots without good surrounding habitat tend to die out in drought years.
Things you can do: Allow small patches of brush every 100yards or so. Half cut the limbs of the brush so it creates an umbrella for loafing cover.Disc some meandering strips periodically— exposes insects and and promotes forbs and grass diversity. Scoop out some small, shallow tanks about the size of a suburban that will catch water. When they dry theywill sprout different plants than the surrounding area. Don’t kill the harvester ants— they promote plant diversity and birds eat them. The only problem ants are fire ants and that because they eliminate other insects. Leave the hawks cats and coyotes alone they keep the smaller predators that prey on quail and usually don’t eat that many quail. Same for most hawks. Keep the invasive grasses such as guinea grass short.. just mow it to a foot or so. Alternate your disc meanders, don’t just redo the same ones. Just about every native brush is of use to quail as long as the density is about 10-25 percent per acre. Plant diversity is good—almost all natives provide food and often at different times. Pray for rain especially spring. Quail need loafing cover, good spring bug hatches and grassy open roosting. Always place feeders near good escape/loafing cover and fence them against larger animals and predators. Light grazing good, overgrazing bad.

We have one or two coveys hammering the feeders now that I pushed up last weekend. I appreciate the information and will do some work to improve the area (disc, half-cut limbs, and small tanks). I just did some quick Google Earth math, and it's more like 80 acres.

stop feeding corn, get a good balacned 18-20 percent feed

I'm not feeding them corn; they get enough of that at our feeders. The feed I get from the local feed store has about 1% cracked corn blended with the seed.