Compared with the increasingly sunny predictions streaming from his competitors, the note Oppenheimer’s Yair Reiner sent clients Friday morning was something of a buzz kill.

“We believe Apple could report in line to slightly disappointing [fiscal fourth quarter] revenue,” he wrote, advising investors to hold off buying shares until after Monday’s quarterly earning’s report.

The focus of his concern: the iPhone.

Reiner’s estimate for Apple’s fourth quarter iPhone sales was already the lowest of all the analysts we surveyed — 6 million, compared with a consensus just over 7 million and a high estimate (from Citibank’s Richard Gardner) of 8.05 million.

But even 6 million, he says, could prove too high, due to what he calls “component/manufacturing hiccups.”

“The first hint of trouble,” he writes, “surfaced during the iPod event on Sept. 9, when Apple implied that ~3.5M phones had been sold with only 21 days left in the quarter. Subsequent checks showed the iPhone 3GS sold out in many markets. Something was clearly preventing Apple from shipping to demand… Consensus estimates imply that 3.5M phones flowed out to customers in the final weeks of 4Q09, which may be too aggressive.”

But Reiner is hardly an Apple (AAPL) bear. The thrust of his advice is to “keep some powder dry” and buy like crazy if the stock price falls.

“We’d use any pull-back as an opportunity to aggressively accumulate additional shares ahead of several near-term catalysts: a potentially gargantuan December quarter for iPhone (assuming no further component issues); re-acceleration of Mac growth with the release of the new MacBook and iMacs; and the likely announcement of the tablet in early 2010.”

Apple is scheduled to announce its fourth quarter earnings after the markets close on Monday, Oct. 19. Tune in here for our analysis of the results.

[Follow Philip Elmer-DeWitt on Twitter @philiped]