iPhone 4 Black Screen w/ Tethered Jailbreak 4.2.1.

[nineday]

iPhone 4 Black Screen w/ Tethered Jailbreak 4.2.1.
I purchased an Iphone 4 MC603B/A that was running 4.3.3 and managed to downgrade it without SHSH blobs to 4.2.1 and jailbreak it by using redsn0w, greenpois0n, and TinyUmbrella to kick it out of recovery mode whenever it got stuck. Finally, it was up and running, but whenever I turn off the iPhone it goes straight into DFU mode. From there, I can only boot it using redsn0w and the jailbreak is tethered because of that. Now, the screen is black even though the iPhone is on. Is there a way to fix the screen problem, keeping in mind that I can't do a hard/soft reset because it goes straight into DFU mode, and how can I untether this jailbreak?

tl;dr
Specs:
iPhone 4 MC603B/A
firmware: 4.3.3 downgraded to 4.2.1
jailbroken: yes, tethered, w/o SHSH blobs
programs used: redsn0w, greenpois0n, TinyUmbrella

Problems:
Black screen but phone is on
Jailbreak is tethered

Solutions:
?????

[ودالأمير]

Turn the firmware4.3.3 to custom firmware by using suitable version of redsnow then give it to the iphone and remember donot use tinyumbrella at allllllllll and if u wont to kick your iphone out of dfu mode use program called ibooty but evert time your iphone turn off u will need to use it.