If the third time is the charm, what is the fourth time? Well, in the case of Mumford & Sons, it’s the time when you try to reconnect with your past while still moving forward. And it proves to be about as difficult and awkward as it sounds. On their fourth full-length Delta, the British folk quartet quite wisely step back from the surging electric-guitar rock of their polarizing and tepidly received 2015 album Wilder Mind. No surprise there. But even though they’ve picked up the banjo, mandolin and acoustic guitar again, Delta is no retreat to the comfort zone of the folk-fest mainstage. Or even a side trip to the blues bar — not an unreasonable assumption, given the album’s title. Instead, this time the Mumfords try to bum-rush the electronica tent by updating their old-school instruments with layers of lush keyboards, flowing beats, strings and edgier sonics from top-flight producer Paul Epworth. One thing that hasn’t changed: The band’s anthemic, earnestly ambitious songcraft — though here it tends to be a bit more introverted and wistful than extroverted and joyous. So the whole thing ends up being a weirdly unsettled hybrid of new and old, big and small, organic and electronic. And while it has its moments of inspired creativity and emotional clarity, it sounds unfocused and indecisive almost as often as the songs play second fiddle to the production. So fans hoping for another work of undisputed beauty and genius from Mumford & Sons will just have to wait to see what the fifth time brings.