concrete
I have not used this one way that I think you are using is it what they call premixed? or pre made?? We dad used to buy the concrete in the bags and get, me to mix it with navajack a ( 3/4" minus, sand and gravel) He told me 4 to 1 was a good mix for gravel/ cement. If I was making concrete pipes I went 3 to 1 used to drive dad nuts. ( I do miss him) Don't use to much water in it for it will slump more that way. I wants to be just wet enought to shovel and for it to stay on the shovel. As for the gravel ( navajack) there would be amounts listed in loose materials for gravels that should be close the to volume that you need. Have you go a gravel sales company around you that could advise. I didn't read the volume that you had. 4" = 1/4 cu ft x width x length.. Will give you cubic feet. Sorry you probably know this but you can almost never over support/ brace concrete. Hope it works out for you it has been 30 yrs since last worked much concrete slabs.