That's 8am that the stores open for iPad pickup and purchasing, actually. They will not sell your preorder, so no worries, if you preordered for in-store pickup you can get it at any time from 8am to closing, no need to get there ar 4am. Don't quote me on this, but I wouldn't be surprised if the long lines from before store opening were gone by 10:30am, so that might be a good time to go in, before the lunch crowd starts lining up.