Visa 489 is a temporary/provisional visa which means after two years you need to apply/convert this visa to a permanent visa. The difference is with visa 190 you are already a permanent resident and entitled to receive certain benefits like medicare. With regards to applyiing for the citizenshep in the future, you do not need to wait for another 4 years to apply, you will be eligible to apply after 4 years staying in NT as long as you are already a permanent resident when you apply and have been for at least a year and you havent stayed outside australia for more than 12mos. I know for sure that with visa 489 you are free to enter and leave aus as many time you like within the visa and of course work/live in NT during your valid visa. You can also bring family here with you.