Sunderland have signed defender Timothee Pembele, midfielder Adil Aouchiche and strikers Mason Burstow and Nazariy Rusyn.
Burstow, 20, has joined on loan from Premier League side Chelsea, having played three games for the first-team.
Rusyn, 24, joins from Zorya Lugansk on a four-year deal for an undisclosed fee, after 21 goals in 49 games.
Pembele, 20, joins from Paris Saint Germain, while Aouchiche, 21, arrives from Lorient - both on five-year deals.
Defender Pembele played 15 first-team games in the French, in addition to time at Bordeaux on loan, and signs on an undisclosed fee.
Former team-mate Aouchiche left PSG for Saint Etienne, before moving to Lorient and has played 86 first-team games in total.
Burstow was at Charlton before moving to Stamford Bridge, where he made his first-team breakthrough earlier this season against West Ham.
Ukranian forward Rusyn also had spells at Dynamo Kyiv and Dnipro, as well as Polish side Legia Warsaw.
Burstow said of his season-long switch to the Stadium of Light: "It's a huge club with a great head coach [Tony Mowbray] and big expectations.
"I'm here to bring goals to the team and keep supporters on the edge of their seats. I can't wait to walk out at the Stadium of Light for the first time after the international break."
Former Welling player Burstow will not be registered in time for Saturday's lunchtime kick-off against Southampton.
Rusyn will travel to Sunderland once the paperwork is completed.
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