I ordered 1/2" plywood for a subfloor. The plywood was delivered when I was away and I seen right away that it is 1/4". At first look,I thought it was cardboard. My plan was to use 1/2 inch plywood over the 7/8" planks. Would it be ok to use this 1/4" plywood over the 7/8" planks. It's an upstairs bedroom that's going to get carpet. I just re-done the bathroom with 1/2" plywood over the same 7/8" planks.

 

It's not pressboard is it?

If it's 3-ply stuff, there could easily be large voids in the center ply. That's a weak spot I can easily see a chair leg crunching through.

 

While your original subfloor would be adaquate for stiffness (assuming it's in good condition)... that 1/4 inch stuff will do nothing for the structural stability of your floor (why bother?). Call the store - have them make it right! Sounds like you're willing to compromise for the sake of expediency..... have patience... it will get done - but do it properly...