LONDON — At the Brit bow of “Terminator 3: Rise of the Machines” July 21, Arnold Schwarzenegger thrilled die-hard fans when he refused to rule out appearing in a “T4.”

Arnie, speaking from the balcony of the Odeon in Leicester Square, hollered his trademark “I’ll be back” to whistles of approval from 5,000 fans.

“Terminator” devotees clambered atop red phone booths to catch a glimpse of their hero, while superfans in cyborg masks armed with autograph books threaded their way to the front to try to say a few words to Schwarzenegger.

He was astounded by the frenzied reception: “It’s such a great crowd here — it’s fantastic to see such enthusiasm.”

Co-stars Kristanna Loken and Claire Danes joined producers Mario Kassar and Andrew Vajna and helmer Jonathan Mostow onstage to unveil the pic.

Brit comic Jonathan Ross, who introduced the celebs, had the crowd in stitches when he joked at Schwarzenegger’s advancing years: “Some of you may think Arnie’s catchphrase is ‘I’ll be back,’ but perhaps it should be, ‘Awww, my back.’ ”

On hand were thesps Rachel Griffiths and Dougray Scott; comic Matthew Kelly; Arsenal soccer supremo David Dein; designer Alexander McQueen; boxer Chris Eubank; Formula One driver Jenson Button.