You can boot your iPhone 4 tethered with OpenSn0w.
But MobileSubstrate Cydia don't function.
You must boot a ramdisk and then you can install apt.

With apt you can install packets.
You can install deb packets with mobileterminal or with a ssh connection, also.
But the most tweaks need MobileSubstrate to manipulation the iOS system.
For more info send me a private message.
For questions send me a private message, also.