There could be trouble brewing in Cupertino, Calif. It has been four days since Apple began accepting preorders for the iPhone 5c. During that time, only the unlocked models have sold out. For the first time, Apple has failed to announce preorder numbers for its newest iPhone.

In a new press release, Apple has announced what we already know: new iPhones are arriving in stores this Friday, Sept. 20. What they failed to mention is how iPhone 5c preorders have gone since they began on Sept. 13.

Apple today announced that iPhone® 5s, the most forward-thinking smartphone in the world and iPhone 5c, the most colorful iPhone yet, will be available to customers on Friday, September 20 at 8:00 a.m. local time at Apple retail stores and at 12:01 a.m. PDT at the Apple Online Store (www.apple.com). iPhone 5s and iPhone 5c will be available in the US, Australia, Canada, China, France, Germany, Hong Kong, Japan, Puerto Rico, Singapore and the UK.

Since the iPhone 4 was launched three years ago, Apple has announced the number of devices preordered during the first weekend. In 2010, that number was 600,000. One year later, iPhone 4s preorders topped 1 million units in a single day. Last year, Apple sold 2 million iPhone 5 units in the first 24 hours.

Apple didn’t offer preorders for the original iPhone, iPhone 3G, or iPhone 3GS.

As we noted previously, Apple’s failure to sell out of iPhone 5c models could be attributed to a very efficient supply chain. That doesn’t explain Apple’s silence when it comes to announcing weekend preorders.

Not surprisingly, all this is causing headaches on Wall Street. Since Apple’s iPhone event on Sept. 10, the company’s stock price is down 9 percent before the start of Tuesday trading.

What’s happening here is really anyone’s guess, at least outside the walls of Apple’s corporate headquarters.

Maybe Cupertino is waiting until Monday, Sept. 23 to announce actual sales for both the iPhone 5c and iPhone 5s. Perhaps, we’re all a little bit trigger-happy and Apple will announce details about iPhone 5c preorders later this week.

With today’s press release, however, that last point is less likely.

Possibly, iPhone 5c preorders are so terrible that Apple has decided to take a wait-and-see approach.

We’ll continue to follow this story. In the meantime, see: It Looks Like Apple’s Gold iPhone 5s Is The Most Popular Of The Three In ChinaSprint Takes $100 Off The Price Of iPhone 5s, iPhone 5c For New Customers, and Tell Them About The New Trends: Apple Airs Colorful TV Ad For iPhone 5c.