WPP Chief Executive Martin Sorrell is one of the world’s most powerful advertising chieftains, but he has a thing about garages.

Speaking at the Reuters Global Technology, Media and Telecoms Summit via videoconference from London on Tuesday, Sorrell expressed concern over new technology and ideas that young unknowns could be toiling on that could completely alter his business.

“I think it was (former Disney CEO) Michael Eisner who said: ‘Six or half-a-dozen PhD students in a shed in Silicon Valley is what worries me most.’ Well, what worries me is half-a-dozen students in a garage in China.”

Sorrell also dipped into a bit of numerology when he responded to a question about whether the world still needs four to six big advertising companies:

“There’s a sort of funny logic to the number four,” he said, noting the existence of four big accounting firms, four big consulting companies and ”four or five” major investment banks. “There is some sort of strange magic to four.”

Those PhD’s in the Chinese garage might be able to explain the magic behind the number four.

Other Sorrell revelations:

- On why WPP’s acquisition plans are modest compared with Google and Yahoo: “If I add up the market caps of the four parent companies in our industry, I get to about $50 billion. If I add all their revenues together… I get to about 33 [billion dollars]. Google has a third of the revenues at 11 [billion dollars] and a market cap three times the size, so they have considerably more firepower. They can breeze in and pay $3.1 billion for DoubleClick. We’re sort of a little bit handicapped in comparison so our ambitions would be substantially more modest.”

- On advertisers experimenting with digital media such as Second Life: “You’re not going to get fired for experimenting and failing in these new areas.”

You want to put that in writing?

(Robert MacMillan in New York and Georgina Prodhan in Paris listened in via videoconference and wrote this together.)