You need a minimum of 4 years uni study in psychology. It doesn't matter if the undergraduate degree is an arts degree or science or whatever, so long as it has psychology as a major. Since most pass degrees are three years, that means doing a pass degree and then an honours year - which is why entry to honours in psychology is very competitive. If you don't get into honours, you need another degree (postgraduate) for the extra year - as a graduate diploma or a masters or something like that. There is then professional training to be admitted as a practicing psychologist.