For a detached garage, do most people go with the 4" form or 6"? How much $$$ is saved on a 1,200 sqft garage by going to the 4" form?

It would probably be a little less money to just stick frame the garage but I figure might as well stick with ICF since the home is ICF.

I'd use a 4" form for interior walls or a doghouse or an attached tool shed (maybe), but that's about all. You're talking about a difference of about 10 yards of mud. One truckload or about a thousand bucks.

Not much difference in total cost of concrete. You will use less, but I always change to a mix with 1/4" rock for 4" forms. In my area this adds about $20 per meter to the mix (less volume, but more cost/unit). The only time I would recommend a 4" form is if space is a premium - if you have room to lose 4" on each length then go to 6" block. Also, if you have a large overhead door that you would like a concrete lintel to span over, this may not be possible with 4" block (depending on load, depth, span, etc.)

With 4" core blocks be prepared to purchase a 3/4" vibrator head for $112 because most probably it will be next to impossible to rent one or find someone who owns one and is willing to lend it to you. I figure at 1200 sq. ft 4" vs 6" comes out to 7.5 cubic yards more give or take. With tax and all that will cost you approx $750 more. The pumper might charge you an extra $100 for three loads with 6" vs two loads with 4". All together you're looking at an extra $1000 that can be easily quantified for 4" vs 6". There may be an extra cost involved with shipping 4" core blocks from very far away. The 4" core blocks are not standard and an extra shipping cost may be added to the cost. This may be in the thousands of dollars depending on where you are and which manufacturer you chose.

I would use stick frame. If you're not going to heat the garage the extra insulation that ICF blocks will give you won't be of consequence. If you want the durability of concrete consider cement blocks.

Regarding the pencil vibrator, what do you recommend and what is the length of the wand?

I will just go with the 6" core. Doesn't make sense to do the 4" core.

You're right, it doesn't really make sense. There is a limit to the reinforcement that you can routinely get inside a 4" envelope. You could build little cookie-cutter homes with it, so that all the little details were carefully worked out in advance, but then you would want to inspect very closely for steel placement and there would almost certainly be adjustments required. Interior walls are less critical, there is less downside to exposure of the steel, nice to have the extra few inches of floor space and the penetrations are not quite as thick, but you almost don't want to have them (4" block) on the site because of the inevitable calculation errors, inventory issues, etc.....

Just much easier to have 6" all around for one-offs and customs.