By Josh Long

T-Mobile USA on Thursday unveiled plans to invest $4 billion to modernize its network and migrate to 4G LTE wireless services as the mobile-phone company fights to become a more serious threat to its larger rivals.

T-Mobile, the fourth-largest U.S. mobile operator, plans to invest $1.4 billion in incremental network improvements over the next two years and launch 4G LTE services in 2013. The wireless operator anticipates that its high-speed service will be available in most of the top 50 markets.

The company cautioned that its LTE plans are contingent on successfully refarming spectrum and getting regulatory approval to receive spectrum from AT&T under the break-up terms of the companies’ failed $39 billion merger.

T-Mobile also said it needs additional airwaves to build a broader and deeper LTE network.

The company has been struggling to compete with larger rivals  namely AT&T and Verizon Wireless  that have been gaining rather than losing subscribers and are ahead in the development of 4G LTE networks. In the fourth quarter of 2011, T-Mobile lost 802,000 customers on contract. The company ended the year with 33.2 million customers, and that’s down from 33.7 million customers at the end of 2010.

T-Mobile is hoping that its investments will allow it to return to growth as the Deutsche Telekom-owned company continues to market itself as a provider of affordable mobile-phone services.

“We want to be known for delivering the best value in wireless because of the advanced technology we deliver at an affordable price,” T-Mobile USA President and CEO Philipp Humm said. “Over the next two years, we’re prioritizing and investing in initiatives designed to get T-Mobile back to growth in the years ahead  beginning with the transformation of our network.”

T-Mobile’s network strategy includes the installation of new equipment at 37,000 cell sites, and the company said it expects to be the first carrier in North American to modernize its 4G infrastructure with new antenna integrated radios on numerous cell towers. The upgrade, T-Mobile said, will strengthen its coverage.

Bellevue, Wash.-based T-Mobile said it will continue to expand its product and service offerings as well as an HSPA+ 4G network that presently covers more than 200 million people.

T-Mobile’s newest smartphone, the Samsung Galaxy S Blaze 4G, will be available for purchase next month.