cannot download an app due to error 404, why?
Also how does anyone contact Apple's app store if the app you purchase doesn't download?
MacBook Pro (13-inch Mid 2012), OS X Mavericks (10.9), SR MBP 2.4ghz, 4MB RAM.
Try re downloading the app. If it's a paid app, you won't be charged again but you must use the same Apple ID.
Downloading past purchases from the App Store, iBookstore, and iTunes Store
If that didn't help...
Disable anti virus software if installed
Make sure the firewall is turned off in System Preferences > Security & Privacy > Firewall
Make sure Gatekeeper is enabled
Open System Preferences > Security & Privacy then select the General tab.
Make sure either Mac App Store or Mac App Store and identified developers is selected. If that area is grayed out, click the padlock icon to proceed.