Sprint and Clearwire have just flipped the on switch to their 4G network in four California cities: San Francisco, Oakland, Palo Alto and San Jose.

The rollout comes after almost two years since the companies’ first WiMAX broadband market launched in Portland, Oregon and brings the total number of markets served to 71. The service is accessible from a range of WiMAX-capable devices and phones, including a QWERTY-equipped variant of the Samsung Galaxy S (Epic 4G) and the HTC EVO 4G, home or mobile hotspots, and data modems for notebook computers.

To help promote the launch new customers who sign up online will receive a 50% discount on their service plan for the first two months, with overnight shipping included and no activation fee. With this, Sprint and Clearwire have completed the first major phase of their nationwide mobile broadband rollout as planned, and prepare to go head-to-head with Verizon, which rolled out LTE service covering a potential 110 million customers in one fell swoop last month.

Meanwhile, AT&T is taking a different approach to ease network congestion in busy metro areas by increasing its Wi-Fi hotzone presence. These hotspots work to complement the carrier’s 3G coverage and are accessible free of charge to subscribers.

The company has already been offering Wi-Fi service around New York City's Times Square and is expanding that zone ahead of the annual New Year's Eve celebration. Other areas where AT&T has established Wi-Fi hotzones include downtown Charlotte, N.C., Chicago's Wrigleyville, and soon San Francisco’s Embarcadero Center.