are a couple of 10' & 8' 2x4's too much $$$ ? plact the conc in sections - 10' x 7'6" w/2contraction jnts @ 90*,,, you could even use 1 x 3's to save on lumber costs,,, i don't know what the 2x4's in the middle mean but neither do you so we're even :beer: you only need to form the slab's perimeter,,, next time you're in an apron/vest store, pick up the basic conc book,,, BIG help seeing pics,,, suggest you get a couple friends to help when its time to place & finish,,, even better if 1 of them knows what to do.
since you mentioned 'screed', that's the 10' 2 x 4 that will ride on top of your level AND square forms which you will saw back & forth to level & place the conc ( you could also call the form a type of 'screed rail' - that's not correct but your 2 x 4 screed will be riding on top of the forms,,, for 4 pours 10' x 7'6, we wouldn't be using a bull float,,, 1 nice thing about smaller pours - IF/when you f-up, you only have to replace a small piece - good luck !