The 22-year-old former Super League young player of the year had been under contract for two more full seasons.
Johnstone, who was called into the England elite performance squad earlier this season, told the club’s website: “It feels amazing.
“I have loved my time here.
“Coming through the academy I always wanted to play for the first team and making that reality has been a dream come true.
“As soon as I got the option to stay for another few years [and] to extend the contract I pretty much jumped at it.
“I can’t wait to see what happens over the next four years.”
News of Johnstone’s extended deal comes four days after Leeds Rhinos confirmed winger Ryan Hall will leave the club at the end of this season.
Johnstone, a product of the Stanningley community club, was many Rhinos fans’ choice to replace the England man, but the extension ends that possibility.
Trinity coach Chris Chester told Wakefield’s website: “It’s fantastic news to get it done so early and credit to Michael [Carter, Trinity’s chairman].
“It’s a massive, massive re-signing for the club. Looking at the squad, we’ve pretty much got everybody who we wanted to tied down to long-term deals.
“The future looks really bright.”
Trinity will secure their place in the Betfred Super-8s if they win at home to Hull on Sunday.