Clearwire says it is gearing up for widespread launches of its 4G WiMAX service in the fourth quarter of this year, expanding into several new markets including Philadelphia and Chicago.

The ambitious wireless broadband provider plans to make its 4G services available in more than 25 markets covering over 30 million people by the end of this year.

Clearwire, Comcast and Sprint each will launch individually-branded 4G service in Philadelphia and Chicago over the next several weeks. In addition, all three providers will begin selling throughout the Seattle area in early December.

Clearwire says it will be expanding its current networks in Philadelphia, Chicago and Seattle as part of the launch. The company also said its customers would be able to buy new 4G devices, but didn’t provide any details beyond USB modems and Intel notebooks.

Separately, Clearwire and Sprint also announced November launches of their respective Clear and Sprint 4G services in Charlotte, Greensboro and Raleigh, North Carolina; Austin, Dallas/Fort Worth and San Antonio, Texas. Service will be available from both companies in Honolulu and Maui, Hawaii, in early December.