Stay 4 consecutive years is to get the citizenship (i.e. passport). After the 5-year PR visa expired and before you become a citizen, you can get the resident return visa (RRV). To get RRV, you must live in Australia in two of the past five years or have special reason.
Since my visa was granted in 2015, if I settle by 2018, I can get the RRV in 2020. In an extreme case if I never need to travel outside Oz once settled, I can settle in 2020, stay till 2024, and get the passport.
Please note you can still legally work and live in Oz after the PR visa expired. The only problem is that if you ever have the need to travel outside Oz, you need a return visa or Oz passport for re-entry.
Some illustrations:
1. Settle in 2015, get citizenship in 2019
2. Settle in 2018, get RRV in 2020 and citizenship in 2022
3. Settle in 2020, get RRV in 2022 and citizenship in 2024. Between 2020 and 2022 you are not allowed to travel outside Oz unless you have special reason.
RRV is for re-entry only. Even without RRV you are still permitted to stay in Oz as long as you settle before the initial PR visa expired.
For all cases, need to stay 4 consecutive years in Oz to get citizenship (of coz you are allowed to travel for short period of times during the 4-year waiting period)