Bootloader unlock is not possible at the moment. Devs with plenty of knowledge are looking into it with various methods. Downgrade to 4.2 isn't possible as you can't flash the older bootloader which is necessary to use the exploit. Safestrap, Touchwiz ROMs, and the one GPE in our general section are the only things we have right now.
Sent from my SGH-I337 running GPE
Current Device: ATT Samsung Galaxy S4 (i337)
Droid Bionic (GSM Hacked)
HTC Inspire 4G (Desire HD)
HTC Dev Phone 1 (Dream/G1)