A 10" round tier that is 4" high? I have never used mixes before, but I am desperate (and out of time!) so I'm trying it. I'd really rather not go to the grocery store. I have 1 box of mix left after my other 2 tiers which are different flavors. Thanks for any ideas. 

I'm not a mix user, but I'm pretty sure you won't be able to.

http://cakecentral.com/a/the-original-wasc-cake-recipe

I know that's a popular doctored mix, and she says it will produce enough for 2 8" rounds. I read through a couple of the extender recipes on here as well, and none of them would produce enough either.

You can do a search for 'mix extender', and a number of recipes pop up, I just scanned a couple, but not all of them.

A1 18 oz. box mix will give you one 2" high 10 inch round. But since they have shrunk the size of the mixes you really need 1 box mix plus extender to give you a 10 inch round that is 2" tall. You'll need another cake mix to be able to do a 4" tall cake