I know the guy who used to be the technical director at Arxx several years ago. He said at that time that they use a lot of four-inch block in homes as described by Kinsey above. I never used it myself.

For above grade work only.........4 inch Nudura :-)
Imho 4inch block is stronger as it has less concrete material weight to pressure the form.
 Less liquid head.
For this same reason it is easier to straighten and brace your wet wall.
Fill material cost is 1/3 lower than 6 inch.Mud has to be plastic.
You capture some extra square footage in floor space every time.

We try to never double up a rebar in a 4inch lintel. If 2 @10mms are called for we go to one 15mm or a 20mm so that concrete placement is not impeded.

Every lintel span and point load has to be thought thru, but that should be done anyway.
We've had to encase steel columns and allow for steel beams in outside walls..but that just makes for more sport :-)

Helps to have a pro P Eng to work with.

Hambro joists on 4 inch work fine also.'Got the t-shirt.

From a design perspective, I'm sure I would flinch putting a 4 inch form across a 10 foot opening with a lot of load. I probably wouldn't use it carrying the joists or trusses, but it should be able to make it across carrying its own weight and a small portion of the building without too much trouble. A concrete wall above the lentil acts as its own being so as long as it's reported during placing of the concrete the lentil itself wouldn't carry much weight. I wouldn't expect to see a large amount of reinforcing and too much could possibly increase cracking. Stirrups would be important. 4 inch forms require a little more detailing and care placing the rebar, but for most residential they should work okay. I would probably use 6 inch myself, but would not try to talk someone out of using four-inch if concrete cost was important to them. Re: monkman, Thanks for the plug for us PEs. Blocks may vary in strength for different thicknesses; I don't know enough about all of them. But the pressure on the blocks is the same regardless of concrete thickness. The "liquid head" has to do with height. You might get a little less force on the walls with certain mixes because of internal friction but that's about it.

Right the "head" pressure with a liquid is strictly based on height not volume. But I have heard that there is less pressure in a thinner wall and believe it. Probably because concrete is not completely liquid and the flow is not as good through a 4".

The 2" of concrete savings would translate to $.60-$.70 sf, but a more expensive smaller aggregate maybe with some admixtures would be required.

Any new comments on 4" ICF use? I was asked to provide an option for it.

I'd use it for a doghouse.
Heated pump or battery shed.
Mother-In-Law of very small size.
Combo greenhouse/shed.
Walk-in cooler add-on.
I'd consider it for non-structural interior walls.
Etc.

I see no advantage to 4" concrete cores. The cost difference in concrete is negligible and trying to run rebar and consolidate in a 4" space is not easy. From everything I read, 4" is the absolute bare minimum thickness of structural concrete.

If concrete costs $82 a yard for a 3,500 psi mix, delivered to the site. Let's run the numbers for 2,500 square feet of wall space:

6" core = 46 yards of concrete = $3,800 of concrete
4" core = 36 yards of concrete = $2,950 of concrete

You saved $850 in concrete. That is basically insignificant when speaking in construction terms. The contractor will probably still charge the homeowner $3,800 for the concrete and pass the savings onto himself.

Generally 4" is primarily used these days when there's a footprint issue.

For the engineers on here.....can you use helex in the 4" and do away with rebar?

Water well house, 6x9 exterior, 6.5' above grade, #4 rebar.  My wife and I did this for the experience.  With horizontal bars centered, the verticals alternating each side, we had about 1.25" cover.