Where can I download iOS 6.x firmware files for the iPhone 4?

Apple made the iOS 6.0.0 update available to current iPhone 4 owners free of charge. If you are looking to download the IPSW files for your device, they are available here:

iPhone 4 (AT&T / GSM) iOS 6.0.0: iPhone3,1_6.0_10A403_Restore.ipsw
iPhone 4 (8GB) iOS 6.0.0: iPhone3,2_6.0_10A403_Restore.ipsw
iPhone 4 (CDMA) iOS 6.0.0: iPhone3,3_6.0_10A403_Restore.ipsw

iPhone 4 (AT&T / GSM) iOS 6.0.1: iPhone3,1_6.0.1_10A523_Restore.ipsw
iPhone 4 (8GB) iOS 6.0.1: iPhone3,2_6.0.1_10A523_Restore.ipsw
iPhone 4 (CDMA) iOS 6.0.1: iPhone3,3_6.0.1_10A523_Restore.ipsw

iPhone 4 (AT&T / GSM) iOS 6.1.2: iPhone3,1_6.1.2_10B146_Restore.ipsw
iPhone 4 (8GB) iOS 6.1.2: iPhone3,2_6.1.2_10B146_Restore.ipsw
iPhone 4 (CDMA) iOS 6.1.2: iPhone3,3_6.1.2_10B146_Restore.ipsw

iPhone 4 (AT&T / GSM) iOS 6.1.3: iPhone3,1_6.1.3_10B329_Restore.ipsw
iPhone 4 (8GB) iOS 6.1.3: iPhone3,2_6.1.3_10B329_Restore.ipsw
iPhone 4 (CDMA) iOS 6.1.3: iPhone3,3_6.1.3_10B329_Restore.ipsw