Barcelona opportunity is still open, but waiting for Real Madrid to lose points in the next four matches is impossible. After losing to Atletico Madrid in the first half of the season so far, no team has been able to beat Real Madrid, Barcelona must do it themselves in the upcoming El Clasico match to reduce the point difference. Finishing in second place in the La Liga standings, and fighting to reach the semi-finals by beating PSG is Barcelona main target now, while Barcelona chances of winning the Champions League trophy also seem very low.
It is not impossible that Real Madrid can lose points in the next 4 matches. It won't be easy for Real Madrid to secure 3 points in each match. They will play against Athletic Club, Mallorca, Barcelona, and Real Sociedad. All of the teams are quite strong, even Athletic Club and Mallorca are the finalist of Copa Del Rey this season. Sure, these are not average teams, they won't reach the final if they are weak teams. On the other hand, Barcelona and Real Sociedad are 2 top teams in La Liga, it will be never easy to get full points against them. So, basically the next 4 matches are tough matches for Real Madrid.
The point gap between Barcelona and Real Madrid is 8 points. With 9 remaining matches, in theory Barcelona still has the chance to overtake Real Madrid. However, Real Madrid must realize it, they will try all out to keep winning and getting full points in each match. As long as they won't lose focus and there is no many injured players, it is very unlikely to see Real Madrid wasting many points. So, I assume Real Madrid will keep leading the La Liga standings until the end of the season although Barcelona can beat Real Madrid in the upcoming El Clasico.