As far as I know there is only one manufacturer who makes a block for a 5" concrete wall. This seems like a bit of an advantage to me. I will be building on a small island 12 miles offshore. There is one sand pit and the aggragate grading is not the best so I am concerned about flow in a 4" wall which doesn't leave a lot of space between the form and the bar. Going to a 6" wall is obvious but my island concrete is about $200/cy so if I can use 16% less I can effect a significant savings. I am I right to be concerned about conslidation in 4" walls and does a 5" wall seem like a good "middle ground"?

You can go with a 4" wall. Choose one that has a minimal web add Helix to cut back on the rebar and go with a mid-range water reducer to increase flow.

Lack of ready available engineering is why I don't like the odd size ICFs. The 4" is okay, but there down-fall is long lintels/openings. Look into composite ICF, they use a lot less concrete but are best suited for stucco.

Between 5 and 6, denpending on the size of the house, the savings on concrete will be minimal compered to the energy savings. By the way 4 inch is hell to pour!

Cheers!

4", 5" its not that big of a differnece. if you know concrete, and engineering, you'll be fine. I have dealt with a number of builfings that use the 4" forms and have no problems, as long as the engineer hasn't overwhelmed the wall cavities with excess rebar ( all depends on calcs. ) Also, take a look at some of the past threads that deal with concrete mix designs to get an idea of what can/might work for you.

If the 4'' cavity isn't overwhwhelmed ,it'll be a breeze. Between 5&6 on a 30x30 is about a truck difference (not much compered to r performance sacrificed).

  We have built and poured a lot of 5" forms this works great.
6" is a great , but you really get the same out of a 5" form,it fills about the same as a 6" and not like a 4".
 We built about a mile of the 5" forms with 6" forms on the same pours.
 The both of them side by side worked the same. we also poured the floor the the one above, monolithic with the 5" interior and 6" exterior walls.
 I would say it is just the right size and the 11" finish size works well.
look it up in the PBM and you can work off the chart.