Bendtner had a chance to add to his tally but lashed wide after he had collected Emmanuel Eboue's scuffed shot.
West Brom's fans taunted him and his Arsenal team-mates by chanting "Champions League, you're having a laugh."
But it was Arsenal who were to laugh last as they took over the game almost completely from that point on.
After Eboue had smashed a shot off target and Arshavin had done similarly, it took a composed finish from Toure to restore Arsenal's lead as he headed in Arshavin's free-kick from the left.
The defender then turned provider, launching a long ball over the top for Bendtner to control with his knee and then crack a volley into the far corner.
But Arsenal's recent profligacy was in evidence after the break when they managed to miss a host of chances.
Bendtner is maturing well - Wenger
Russia international Arshavin should have opened his account but was twice denied by Scott Carson, after Bendtner had created the chances.
Substitute Abou Diaby was next to take aim, but his shot was deflected over and shortly after Eboue shot straight at Carson from 12 yards out.
In between those opportunities, Bendtner came within a whisker of completing his hat-trick when he drifted across the edge of the penalty box before striking a low shot that cannoned back off the post.
By that stage West Brom had long looked a beaten side but they might have given themselves hope late on had Marc-Antoine Fortune squared the ball to an unmarked Moore rather than fire harmlessly across goal.
Arsenal missed yet another chance for a fourth goal late on when Arshavin's shot from inside the area was well saved by Carson.
West Brom manager Tony Mowbray:
"We won't win matches if we defend like we did in the first half.
"The defending has got to improve. I take responsibility for the fact I bought all the players in, but the defenders are not stepping up.
"I am frustrated. We work on it - we don't just wash things away. I have changed the individuals, but the goals are still going in against us."
Arsenal manager Arsene Wenger:
"Bendtner is improving from month to month. He will be better every three, six months.
"He has shown tonight he has the talent to be an Arsenal player.
"That is the right way to mature. I liked his presence. I liked his determined attitude and the way he took people on. That shows he has improved."
Carson, Zuiverloon, Meite, Donk, Robinson, Morrison (Menseguez 72), Greening, Koren, Brunt (Filipe Teixeira 72), Moore, Fortune.
Subs Not Used: Kiely, Hoefkens, Cech, Kim, Borja Valero.
Koren, Brunt, Meite, Donk.
Almunia, Sagna, Toure (Diaby 46), Djourou, Clichy, Eboue (Ramsey 67), Song Billong, Denilson, Nasri (Merida 83), Bendtner, Arshavin.
Subs Not Used: Fabianski, Van Persie, Vela, Gibbs.
Bendtner 4, Toure 38, Bendtner 44.
Steve Tanner (Somerset).
BBC Sport Player Rater man of the match
: Arsenal's Nicklas Bendtner with 8.25 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.
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