A step backward?
It's conceivable that this might be just a statistical fluke -- 4% isn't a very big change -- but that's probably not the case, because other results are in line with the change. The percentage of those who say they would be uncomfortable at discovering a family member is gay rose 3%, the proportion of gay people reporting encounters with discrimination is up 11%, etc. These figures represent something real.
Confronted with evidence of inexorable social progress, the enemy likes to use the metaphor of the pendulum -- claiming that while attitudes may move in one direction for a while, they inevitably "swing back" to the more conservative position, and the midpoint of the swings doesn't really move. However, there's no evidence that changes in public attitudes on social issues really work like that. The success of the Nineteenth Amendment was not followed a few decades later by a mass shift of public opinion back toward the view that women shouldn't be allowed to vote after all, much less repeal of the Amendment. The Civil Rights movement wasn't followed by a "swing back" to majority support for the KKK and Jim Crow laws, much less reinstatement of those laws. The sexual revolution didn't end in a full-scale return to the values and behaviors of the fifties. In each case there was some degree of backlash, yes, but except among lunatic-fringe elements, it never resulted in a full return to the attitudes of the previous status quo, much less actual reinstatement of the previous customs and laws. Not even close. There really is such a thing as a continuing trend of progress, even if temporary setbacks occur.
The linked report says the findings "show that the attacks on the community by the Trump administration are having a real effect" -- that is, that the decrease in acceptance is being caused by the more openly hostile stance of the government. This, too, seems unlikely. Is it really plausible that 4% of the US population, having previously felt accepting toward gays, has been persuaded by the Trump regime's (rather muddled and sporadic) anti-gay rhetoric that their tolerant stance was mistaken and that they should be more hostile? It's hard to see how that would work.
I think what the findings represent is not a genuine increase in prejudice, but a more open expression of prejudice which was already there. Most readers probably know of the "Bradley effect" -- even when speaking to anonymous pollsters, some people are uncomfortable declaring views they actually hold if those views are perceived by the larger society as being disreputable. As homophobia has become increasingly marginalized over the last couple of decades, those who remain prejudiced have become less comfortable expressing it -- in some cases, even in anonymous surveys.
The difference Trump's rise to power made is that bigoted views of all kinds are now regularly emanating from the apex of the government, which means that their perceived disreputability and marginalization has decreased somewhat. We've already seen the effects of this with racism, which has been manifesting itself much more openly since the 2016 election, with increased activity on far-right websites, the Alt-Right march and violence in Charlottesville, etc. There's no evidence that this is happening because Trump's victory made more people become racist, and in fact the identifiable individuals involved in these activities have histories of racist views going back long before 2016. It's not that there are more of them, it's that they're emboldened.
In the case of the 4% drop in the number of people who are accepting toward gays, I think that this 4% represents those who really weren't that accepting in the first place, but claimed in surveys that they were, because they knew their actual views were socially frowned upon. Now, thanks to Trump and the policies he's pursuing, they perceive their views as more mainstream and thus feel free to express them.
One corollary of this is that, once a progressive stance on an issue gains an aura of mainstream respectability, surveys are probably overstating actual mass support for that view by just a little. That is, the increase in support for gay equality we've seen over the last few years is real, but the surveys each year are probably running a couple of points ahead of the reality.
There is no pendulum swinging back. All Trumpism has done is to reveal that the express train of progress has not moved quite as far toward its destination as we thought it had. It is still moving, and it will still get there.