The first spacecraft in the series is scheduled to launch this year while the fourth is not due until 2016, the aerospace and aviation giant said.
Global Xpress is intended to provide Inmarsat's government and commercial customers with mobile broadband connectivity on land, at sea and in the air.
"Receiving this option to our initial three-satellite agreement with Inmarsat positions Boeing as the key manufacturer of this vital communications network," said Craig Cooning, vice president and general manager of Boeing Space and Intelligence Systems.
"We appreciate this vote of confidence and will continue to live up to it as we look forward to the first satellite's launch later this year," Cooning said.
Scheduled to be delivered in 2016, the fourth satellite will be based on the Boeing 702 High Power platform. Boeing and Inmarsat signed the original Inmarsat-5 agreement in 2010.
Earlier in October Inmarsat Government, a major telecoms supplier to the U.S. government, said its new L-band tactical satellite solution had been tested and acquired by the government, enhancing military communications capabilities.
L-TAC complements the scarce UHF satellite capacity with a single-hop, low-latency voice and data service, providing tactical L-band service when UHF channels are inaccessible or unavailable.
"Military users need to exercise reliable and cost-effective command and control of widely dispersed forces in austere environments where terrestrial infrastructures are non-existent," said Susan Miller, Inmarsat Government's president and chief executive officer.
"The L-TAC service provides clear communications at high speeds, crucial for convoys on the move in hostile environments. Such service is in critical demand on a global basis, enabling existing in-service tactical radios to access beyond-line-of-sight communications without the need to modify existing hardware."
Immarsat gave no additional details about the system or its acquisition by the government.
Boeing Defense, Space & Security is one of the Boeing Co.'s major divisions and one the world's largest defense, space and security businesses specializing in innovative and capabilities-driven customer solutions.
Boeing Defense, Space & Security is a $33 billion business that employs 58,000 people worldwide.
Inmarsat is in the business of fulfilling government requirements for voice, data and video services worldwide.
The company owns and operates three global constellations of nine satellites flying in geostationary orbit. These will be joined shortly by the powerful Alphasat I-4A F4 spacecraft and, from 2013-14, by the Inmarsat-5s to power the new super-fast Global Xpress network.