I agree with the exchange risk, I think 4 weeks will deter them enough. We can also blacklist exchange accounts from voting as part of steem chain policy. We can't prove whether 4 weeks will stimulate investment, it is a hypothesis, only by testing and experimenting can we prove or disprove the hypothesis and adjust accordingly. 4 weeks is a good starting point.
RE: HF Proposal: Vote to Reduce Power Down Period to 4 Weeks