The HF will mean a more powerful vote, but it won't be 4 times more powerful, at least not in terms of price.

It will be 4x more powerful in terms of the number of rshares you're contributing to the post, but that won't result in 4x the payout because other voters are contributing a lot more rshares as well. The size of the reward pool isn't changing, so when more rshares are chasing the same about of money, the value of those rshares decrease. Henceforth, you're contributing 4x the rshares per vote, but those rshares do not result in 4x the payout.

How much the value of a vote will increase is hard to figure out. It will be less than 4x, but more than it is now. My best guess would be somewhere around 2x. That's assuming all else remains equal. Will the no-vote-for-whales pact stay around or will that go away with the new rewards curve? If whales start voting again, we might even see a drop in the amount our vote is worth.

Here's a good post from 8 months ago explaining the upcoming changes (although, I don't agree with everything he says). https://steemit.com/steem-help/@sigmajin/the-tale-of-the-5-brothers-a-voting-power-parable