If Clearwire’s fourth-quarter results are any indication, things seem to be turning around for the mobile WiMAX company.

Clearwire’s net adds hit 87,000 in its fourth quarter, more than it added during its entire 2008 fiscal year. The fourth-quarter increase included the addition of about 90,000 net new retail subscribers in the company’s mobile WiMAX markets, which were partially offset by slight declines in its domestic and international legacy markets.

Clearwire’s subscriber base is comprised of 46,000 wholesale customers and 642,000 retail customers, bringing its total customer base to 688,000.

The company’s net adds weren’t the only thing to increase. Sales were also on the rise, hitting $79.9 million in the fourth quarter compared to $20.49 million last year. The sales increase helped to offset the company’s heavy spending on its network build, narrowing its losses to $98.73 million from $118.02 million last year.

Clearwire CEO Bill Morrow was optimistic about the company’s wholesale services, whose customers include investors Sprint, Comcast and Time Warner Cable. He also expects the company’s network expansion to triple its 4G subscriber levels over the next year. Clearwire plans to cover 120 million people by the end of 2010, up from 44.7 million by the end of 2009.

Retail ARPU, currently at $39.86, is expected to remain flat in 2010.The company expects to spend between $2.8 billion and $3.2 billion to build out its mobile WiMAX over the next year.