can i just download ios 6.0 for iphone 4 and not install

You can but only through iTunes on your computer. Open iTunes and then connect your phone. Once it recognizes it, you should be prompted to upgrade. Then it will allow you to download and install, or download only.

No way to download only via software update on the phone.

Hope this helps.

Here is iOS 6 for iPhone 4 GSM directly from Apple. You can download to your computer or wherever you like. If it becomes ZIP file, just .ipsw at the end to make it IPSW.

http://appldnld.apple.com/iOS6/Restore/041-7175.20120919.wvv7Y/iPhone3,1_6.0_10A 403_Restore.ipsw