iReb 4.0x, 4.1, and 4.2.1 RC3 are specific versions of the iReb tool. These versions are designed to support iOS 4.0x, 4.1, and 4.2.1 firmware, respectively. RC3 stands for "Release Candidate 3," indicating that these versions are near-final releases, with only minor bugs or issues remaining to be addressed.
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