In my backyard i want to add a little more concrete to the slab i have now.. its 135sq.ft x 4" thick... how hard would it be to just do it by using the mixing bags?
The hard part is mixing all those bags. You can make that job a bit easier with a rented mixer or build yourself a mixing box for the job, which would be better and easier than using a wheelbarrow.
Keep in mind that a 60 pound bag of redimix only covers about 1/2 a cubic foot, so if you've got much to do you're looking at a serious number of bags and a whole lot of work to mix it.
I have seen them before I believe they hold one yard of concrete so you will be able to tow it with a car.