T-Mobile USA tonight is launching a new slogan -- and a new attitude -- with an advertising campaign that lays claim to "America's Largest 4G Network" while poking fun at the notorious network problems experienced by iPhone users on the competing AT&T network.

The new T-Mobile ad, a clear spoof of Apple's Mac campaign, features a woman introducing herself as the new T-Mobile myTouch 4G phone, and a man introducing himself as the iPhone 4 -- with a large, balding man representing "the old AT&T network," literally hanging on his back. The spots start airing tonight on major cable stations and broadcast networks, in addition to national Internet properties.

The initiative marks a shift in T-Mobile USA's mentality to "much more of a challenger brand," said Reid Walker, the company's vice president of communications, via phone this afternoon. The Bellevue-based company, the fourth-largest U.S. mobile operator by subscriber count, recently installed a new CEO, Philipp Humm, and the new leadership appears more willing to take risks and embrace the role of scrappy underdog.

The "largest 4G network" claim is a sign of that new attitude. HSPA+ technically isn't considered a 4G network in the industry. Previously, T-Mobile has described its HSPA+ network as offering "4G speeds," but it has stopped short of calling it a 4G network.

However, a recent assessment from the International Telecommunications Union found that the current WiMAX and LTE standards don't technically qualify as true 4G technologies, even though they're marketed as 4G networks. In essence, rather than suggesting its rivals back down from their 4G claims, T-Mobile has decided to join them.

"If you look at how consumers are viewing 4G, it’s all about speed," Walker said. "When they see some of the current claims out there, they believe they’re getting the fastest phone on the market, which is not always the case, as you look at some of the competitive analysis in the market."

T-Mobile claims theoretical peak speeds of 21 megabits per second on its HSPA+ network, with plans to bump that to 42 mbps next year. By comparison, Sprint claims peak download speeds around 10 Mbps for its 4G mobile service, powered by Kirkland-based Clearwire's WiMAX network.

In conjunction with the new campaign company is launching two new devices, the myTouch 4G Android phone and a 4G netbook, the Dell Inspiron Mini 10 4G. With the expansion to six additional markets today, HSPA+ network is currently in 75 cities, and the company says it will cover markets with total populations of 200 million people by the end of the year.

In a news release, T-Mobile USA's chief technology officer, Neville Ray, says the HSPA+ network "is delivering 4G speeds that match and often beat WiMAX and are comparable to what early LTE will deliver."

Asked if the company was concerned about competitors challenging T-Mobile's 4G assertion, Walker said, "We’re confident in our claim, we’re confident in our positioning, and we’ll see what happens."

In a news release, T-Mobile quotes Chris Nicoll, a distinguished research fellow with the Yankee Group, saying, "Consumers do not understand the technical alphabet soup of technologies involved in 4G, but for our purposes we define WiMAX, LTE and HSPA+ as 4G technologies."

Update, 11 p.m.: For the record, we asked AT&T if it wanted to comment on or respond to the T-Mobile ad, and the company declined.

Update, Wednesday morning: Sprint 4G president Matt Carter responds to the 4G claim, telling FierceWireless: "Halloween is over--it's time for T-Mobile to stop dressing up like their favorite super hero--Sprint 4G."

Update, Wednesday afternoon AT&T disputes 'largest' part of T-Mobile's 'Largest 4G Network' claim