PHILADELPHIA — The Boston Celtics started slow but got a massive boost from Payton Pritchard and their bench to build a big early lead and nurse it the rest of the game. Jayson Tatum and Jaylen Brown got going in the second half to help Boston get their lead up as high as 30. Joel Embiid returned but it didn’t help much as the Celtics won Game 4  128-96 to take a 3-1 series lead.  Here are five reasons they won.  Payton Pritchard Was Cooking  He dropped a career-high 32, but for a while in the first half, he was nearly matching whole starting five’s point total. He was on fire from three, talking his trash and carrying the Boston offense until Brown and Tatum caught up. Brown didn’t score his first points until the second quarter, but it didn’t matter because Pritchard was on what we’d call a wicked heatah.  Derrick White’s Defense Was Impeccable He was still a little hesitant with his jumper, but they didn’t need his scoring.…