President Donald Trump announced Wednesday he has selected Robert C. O’Brien to replace John Bolton as national security adviser.

“I have worked long & hard with Robert. He will do a great job!” Trump tweeted from California, where he is holding fundraising events.

Trump would be the first president to have four national security advisers in his first term.

O’Brien is currently the special presidential envoy for hostage affairs at the State Department. He was nominated by President George W. Bush to serve as a U.S. representative to the 60th session of the U.N. General Assembly, where he worked with Bolton, whom Trump fired last week.

He served as co-chairman of the State Department’s Public-Private Partnership for Justice Reform in Afghanistan in the Bush and Obama administrations.

O’Brien was dispatched by Trump to Sweden try to secure the release of rapper A$AP Rocky, who was jailed without bail in Sweden on assault charges.

Swedish prosecutors wanted Rocky, whose legal name is Rakim Mayers, to serve six months in jail but ruled in August “the assault has not been of such a serious nature that a prison sentence must be chosen,” Stockholm judges said in a statement.

O’Brien also served as a senior foreign policy adviser to former Wisconsin Gov. Scott Walker’s 2016 Republican presidential campaign and to Sen. Mitt Romney’s presidential campaigns.

Rachel Oswald and Clyde McGrady contributed to this report.