Opinium have only conducted one previous voting intention poll over the course of the EU referendum campaign, and it showed a razor-thin Leave lead, so their new poll showing a four-point Leave advantage doesn't particularly indicate that anything dramatic has happened. But assuming there haven't been any methodological changes, it further strengthens the evidence that Remain haven't made progress of any significance in the wake of the Brussels attacks.
Should the United Kingdom remain a member of the European Union or leave the European Union?
Remain 39% (-1)
Leave 43% (+2)
The poll was commissioned by the Observer, who focus on the very familiar pattern of younger respondents backing Remain but being much less likely than their Leave-leaning seniors to actually cast their vote. Irritatingly, though, we're not told whether the poll is filtered/weighted by turnout - if it is, the disadvantage for Remain is already factored into the headline figures. If not, the poll could be implying that Leave are really ahead by a fair bit more than four points. We'll presumably find out one way or the other when the datasets emerge.
Assuming that differential turnout will on balance work in Leave's favour (which it probably will, although ComRes take the opposite view), it's worth pointing out that both polling evidence and past precedent suggests that we're heading for a comparatively low poll in June. Turnout in the 1975 Common Market referendum was around 10 points lower than the norm for general elections of the period, so if history repeats itself we might be looking at a figure in the mid-50s, leaving plenty of scope for the greater motivation of Leave supporters to swing the balance. That said, the result in 1975 was already a foregone conclusion by the time referendum day came around. If the polling this time remains tight and/or contradictory, that in itself could galvanise Remain sympathisers into turning out (in much the same way that No voters were galvanised in the closing stages of the indyref campaign).
I'm going to hold off from updating the Poll of Polls, because there's been a flurry of half-breed polls published recently, and it's becoming increasingly difficult to decide which polls should be classified as 'proper' voting intention polls. Hopefully the mists will clear shortly - but basically, any average of online polls would show a more or less deadlocked race, while telephone polls are for the most part continuing to show Remain ahead, but by a narrower margin than we've been used to.
UPDATE : The Opinium datasets have now been released, and as Roger Mexico points out in the comments section below, not only is there no turnout filter, but respondents who are not even eligible to vote have been eccentrically left in the headline figures. Taking the elementary step of removing those people isn't quite enough to change the rounded voting intention result, but the unrounded numbers work out as -
As usual, the Scottish subsample is far more Remain-friendly : Remain 53%, Leave 30%.