I have an iPhone 4 which i bought from UK and then i came back to my home country and to use it on other networks I was using it with gevey sim. A few weeks ago I upgraded my iOS to 6.1.3 and since then my phone got locked and then i found out that the gevey sim does NOT support 6.1.3.
my modem firmware is 4.12.05

now i am a bit of a noob and i have 2 few questions i want answered so if anyone can please help me:
1. i have looked around and i have found that (maybe) there is no jail break for 6.1.3 on the baseband 4.12.05 so can i downgrade my iOS/baseband?
2.my iphone is prompting me to upgrade to 7.0.4 so if i do what are my chances of using it like before?

i am from Bangladesh and i don't own a credit card and my contract with my carrier provider is also over so i can't buy imei unlock for my phone. currently i am just using it like an iPod and i would really appreciate the help.