There are a few things to keep in mind about all this.
The first is that it was just one poll. And they didn't say who it was. Additionally, there's a margin of error in all this, and "4 points" can be closer to 7. And that early 12 points could have been closer to 10. So, the two numbers may not be as far apart as they seem.
Also, we don't elect people by generic vote. Clearly any margin here is a solid indication of Democratic support, but in the end voters go into a booth to chose a person. Moreover, this was a national poll, and it doesn't take districts into consideration. For all we know, this poll merely shows that the electorate in heavily Red states got more protective towards GOP candidates they were likely going to vote for anyway. It may not be that, but with any national poll it must always be considered. Ultimately, though, what's most important in this coming mid-term election is not what happens in the most-Blue or most-Red states, but in districts that are toss-ups or even just lean a little Red. That's because in the end, Democrats only have to turn 24 districts, not 248.
But there are two other far-more important issues that impact these conflicting news stories.
The first is that I am absolutely, 100% convinced that the most important factor in the mid-term election is enthusiasm, voter intensity, get-out-the-vote. Mid-term elections notoriously have low turnouts. So, even a slight increase in turnout by a party can be massively significant. And right now, it's pretty clear that anger by Democrats and their voter intensity is incredibly high. And Republicans by comparison are far-more ambivalent (some, I dare say, are embarrassed by their party). There definitely are intensely-supportive Republicans, as rabid as Democrats, but those tend to be die-hard Trump Forever voters of the base -- and not only are their numbers small compared to the rest of the full party, but...the focus of their love and intensity -- which is Trump -- is not on the ballot. So, getting these people out to vote in an election where the reason for their enthusiasm is not running is going to be a challenge. And even more is the added factor, related to this intensity, that for the past 16 months, Democrats have been organizing to get people out on the street, putting together marches and protests in local districts at town hall meetings, all of which has not only produced substantive results, but databases of politically-active voters. Add to that the organizing efforts by the "March for Our Lives" people, and you have two notable elements to elections that create incredibly strong starting points. So, what a poll says people prefer, even a poll that favors Democrats, is a distant secondary to people who are enthused enough to actually go out and vote in a mid-term election.
And the second important issue is...reality. Polls aside, if you look at all the national and many state elections that have taken place over the past 6-12 months, we've regularly seen Democrats increase their vote over Trump's totals in the 2016 election by 15-20 points in almost all of them. No, not all. And no, Democrats didn't win all -- some races were just SO heavily Red that winning them were near-impossible. But in most of these races (even where Democrats still lost), that was the margin. Whatever polls say, that is reality. And if you can pick up 20 Democratic points in a district that is so profoundly Republican, that alone speaks volumes about the electorate. Indeed, keep in mind that (as I said) Democrats only have to pick up 24 seats to gain the majority in the House of Representatives, and there are already 18-20 districts with Republican congressmen that Hillary Clinton actually won in 2016. Even if we just consider conservatively that the swing is "merely" 10 points for Democrats (and not that 15-20), the result is career-changing. And now, also add in the 20 or so others districts that Republicans won by only eight points or less, then that same 10 points (again, being conservative, since it could be as high as 20 points in such purple districts) puts all those races into such high risk for Republicans that we've seen 37 GOP congressman already say they won't be running for re-election. Including Speaker of the House Paul Ryan.
None of this means that Democrats should take anything for granted. So much can change in politics in a day, let alone six months. But given the results of the last election, I don't think that will be an issue for Democrats. All it is meant to say is that the voter intensity of get-out-the-vote and the reality we have been actually seeing in elections top the results of any poll. Even polls already in your favor. And both of those, far more than anything else, heavily favor Democrats.