SIL: Include SIL4 to safety levels
SILs are at the moment only from SIL1-SIL3.
Extend SILs to SIL4.
Test: Open requirement, set safety relevant. Choose IEC 61508, now you can choose SIL4.
(from redmine: issue id 2148, created on 2014-11-03, closed on 2015-01-12)