Per Rob Beasley in the Sun, the quartet's futures are all up in the air, with a source indicating that their respective long-term contracts will do little to keep them at the club:
"Yes, they’re tied into deals but those contracts won’t stop rivals moving in at the end of the season. And they won’t stop our players considering their futures either. Contracts don’t count for a lot these days."
Courtois, Oscar and Costa are all contracted to the west London club until 2019 and Hazard until 2020, but the Blues' nightmare 2015-16 campaign has them all considering moving on.
The poor results see them 14th in the league—and likely to miss out on Champions League football next season. This has led to questions over the dressing-room atmosphere and uncertainty over the next long-term manager at Stamford Bridge to leave Chelsea "fearing an exodus," per Beasley.
The loss of Hazard would be a big blow. He has had a terrible time of it this season for Chelsea—he has yet to score a goal—but he is one of their star players, having been named the Premier League's Player of the Season in 2014-15 when the Blues won the title.
He has been widely linked with a move to Real Madrid in the summer, but Los Blancos' imminent transfer ban is likely to rule out a move to the Bernabeu, per Bleacher Report's Sam Tighe and Dean Jones:
However, Ligue 1 giants Paris Saint-Germain could yet give him a route out of west London, having been linked with the Belgian in the past, per the Daily Mail's Neil Ashton.
The Parc des Princes could also be the destination for Courtois, per Beasley.
Chelsea will fight tooth and nail to keep the 23-year-old, though, as he is set to be their No. 1 for years to come.
Furthermore, the Blues' decision to sell Petr Cech to Arsenal last summer means that Chelsea would have to shell out to replace Courtois should he leave, as current No. 2 Asmir Begovic is arguably not good enough to fill the starting role permanently at Stamford Bridge.
PSG have been linked with a move for Costa—for whom a return to Atletico Madrid has also been mooted—with centre-back David Luiz suggesting the Spaniard would fit in well, per Adrian Kajumba in the Mirror:
Diego is one of the most clinical strikers in Europe, he would be a welcome addition to any team. I think PSG have made their intent clear over the last three or four seasons, that we only add the best player. If Diego decides he has had enough of England and wants a new challenge, of course he would be welcome here.
Chelsea's fall from grace has been a spectacular one, and the 2015-16 season will be one to forget barring a miraculous late recovery.
They have the quality and resources to bounce back but must keep hold of their best players to do so.
A clear-out of some players seems inevitable, but Hazard, Costa, Courtois and Oscar all still have much to give and are four players Chelsea need to keep if they are to return to the top of English football.