Sir Martin Sorrell is the kind of founder who people in Silicon Valley most prize. He has enjoyed huge success, having built the world’s biggest advertising conglomerate over 32 years, WPP. He’s also out for revenge. Soon after WPP’s board began investing an “allegation of misconduct” in the spring of last year — it later asked him to pay back $200,000 in personal expenses — Sorrell left the company in a huff. Six weeks later, he’d formed a new company, S4 Capital, using a playbook that he knows works.