The high-level exodus continues for the flailing HTC as COO Matthew Costello has stepped down.

Bloomberg reports that Fred Liu, president of engineering and operations, will have his duties expanded to cover the responsibilities of the departing Costello. This makes four senior executives who have left the company in the past two years.

The Taiwanese handset maker has fallen on hard times in the global smartphone market. At the end of 2012, HTC controlled 4.6 percent of the total market, down from 10.3 percent at the end of 2011, according to recent numbers from IDC, which estimates that HTC shipped 32.6 million smartphones in 2012.

But things could be looking up for HTC if its May sales numbers are any indication. The company posted revenues $971 million for the month, which is still down 3.35 percent year-over-year but up 48 percent over April’s revenues.

The company is leaning heavily on its flagship, the lauded HTC One. The handset has garnered generally rave reviews and now has the support of all four major U.S. carriers, with Verizon Monday announcing it would carry the One beginning this summer.