The SIBsim4 project is based on sim4, which is a program designed to align an expressed DNA sequence with a genomic sequence, allowing for introns. SIBsim4 is an extensive rewrite of the sim4 program to improve execution speed and provide more informative output.Changes: This release introduces a new command line switch, -S, which allows one to better fine tune how insertions/deletions are evaluated around potential splice junctions. A few problems, where spurious very small exons or large gaps in exons were incorrectly reported, have been fixed. The code was cleaned up to silence new warnings from the current GCC releases.