I am for reducing it to 4 weeks only because all SMTs will follow it until custom power down for SMTs is implemented otherwise I believe it won't bring any real benefit for the risks already mentioned by others.
Ideally I want users to stake as long as they want but there must be incentive to stake for long aside from extra security. Here's my proposed implementation from my discussion with @therealwolf from some time before.

Instead of higher APR from staking for longer period, what do you think about witness and SPS voting power multiplier?
For example we can stake for 1 week, 2 weeks, 3 weeks, and so on with upper limit.
STEEM staked for 1 week has x1 voting power multiplier for witnesses and SPS
STEEM staked for 2 weeks has x2 voting power multiplier for witnesses and SPS
STEEM staked for 3 weeks has x3 voting power multiplier for witnesses and SPS
up to the upper limit
This will be very simple to implement (at least compared to separate inflation pools or multitiered privileges you propose above and I proposed before on thecryptodrive's post) and comes with completely custom stake periods. Power down will need a rework like example below:
An account has 100 STEEM staked for 2 weeks and 20 STEEM staked for 3 weeks, they start a power down.
After 1 week passes since power down is initiated the account will have 100 STEEM staked for 1 week and 20 STEEM staked for 2 weeks.
After 2 weeks passes since power down is initiated the account will have 100 liquid STEEM and 20 STEEM staked for 1 week.
After 3 weeks passes since power down is initiated the account will have 120 liquid STEEM

This implementation allows minnows to have a say on governance if they are willing to stake for a long time while whales that only seek to milk the system won't dare to stake for long. The maximum duration exists to prohibit people to stake for like 100 years to gain ridiculous vote multiplier, just set the maximum to 104 weeks (or just 100) like the old powerdown timer.
When thinking about combining it with burn instant powerdown @theycallmedan proposed, the burn fee can scale with the stake duration (ex: 1 week stake have 1% fee for instant powerdown, 2 week stake have 2% fee, and so on)
This implementation is definitely way more complex than just changing a variable but I believe it's win-win for absolutely everyone except abusers.