The Express & Star revealed earlier this week the Baggies had agreed to pay the full £2million required to free the 45-year-old from his contract at Barnsley.
And now Ismael has been confirmed as boss with the length of his deal signalling the start of a new era for the club.
Albion's search for Sam Allardyce's successor had been a protracted one.
A move for Chris Wilder was vetoed by owner Guochuan Lai.
The club then entered advanced talks with David Wagner only for the German to join BSC Young Boys in Switzerland.
Ismael, however, became the number one target for chief executive Xu Ke – who is known as Ken – after former sporting director Luke Dowling left the club.
And the CEO says Ismael has been brought in with a long-term plan.
“I am delighted to welcome Valerien to The Hawthorns," he said.
"Valerien has established a reputation as an excellent coach and I have been impressed by his professionalism, enthusiasm and hunger to succeed.
His performance last season with Barnsley, and his coaching career to date, all point to a man who has a long, successful career in front of him.
"It is my hope that he is with us for at least the next four years.
“I am confident that by working together we will be able to achieve our goal of returning to the Premier League, but success is earned by building on strong foundations, and our plans must now look further into the future.
"For too long we have focused only on what is immediately in front of us. This means we have often neglected our long-term aspirations."
Ismaël added: “It's a big commitment from the club to want me to implement a philosophy I want for the next four years.
"My first task is to implement a new identity that the fans can enjoy and one where they can see the players are there for each other and give everything on the pitch to win games.
"We have a clear plan and vision of where we want to go.
“We’re going to need everyone, especially our fans, to support the team.
"It's a new start for everyone and it’s important that we take on this new challenge together, and we’ll continue to work hard to make the fans proud this year.”
On Albion’s current squad and player recruitment, the former Barnsley boss said: “We have a good mix of young and experienced players in this squad and for sure we will need some new faces in certain positions - but the only massive change here will be the way we play and the formation.
“We are very happy with the group here and now it’s about working on the motivation and mentality of the boys.
“We have the quality but the big difference in this division is the mentality you need. It’s up to me and my staff to create the right mentality for the players and they need to quickly prepare for the Championship and be ready to compete.”