Celtic went ahead after 28 minutes when Arne Engels sent Nick Pope the wrong way from the penalty spot after being brought down by Joe Willock inside the box.
Keeper Pope made a major error to gift the hosts a second just before half-time, rushing off his line and failing with an attempt to head a long ball clear to allow Johnny Kenny to pass it into the empty net.
Newcastle brought on Anthony Elanga, who joined from Nottingham Forest earlier this month for £55m, for his debut at the break, but the winger could not make an impact.
Celtic added a third seven minutes after the restart when South Korean Yang Hyun-Jun slotted past replacement keeper Martin Dubravka from close range.
Despite Newcastle having more possession in the second half, they conceded a fourth goal as Liam Scales powered a 75th-minute header into the top corner.
The Magpies face Arsenal in their next pre-season game on Sunday, 27 July, in Singapore (12:30 BST).
On whether Isak will be part of the tour, boss Howe added: “Yeah, absolutely. He’s part of our squad. At this moment in time, I expect him to come.”
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