This is one of those 'how long is a piece of string?' questions.
Offenders sentenced to IPPs can't be released until they're regarded as no longer posing a threat to society. In order to achieve that status they have to satisfactorily complete lots of courses. Unfortunately the present overcrowding in the prison system can mean that it's many months (or even years) before a prisoner is sent to a prison that can actually provide those courses.
See the 'comments' section, here, for some experiences of those with family members serving IPPs:
http://www.socialaffairsunit.org.uk/blog/archi ves/001471.php
With regard to your actual question, all that can be said is that if your family member is a model prisoner
and he's fortunate enough to be in the right place at the right time (to get on the appropriate courses) he
might only serve 4 years. The experiences of others though suggest that it's more likely that he'll be in prison for at least 5 years and quite possibly for many years longer.
Chris