The fourth inning concluded Tuesday with a standing ovation from much of the small crowd at Citi Field, perhaps wondering if what it just witnessed was real.  But this wasn’t a mirage. The Mets sent 10 batters to the plate in the inning and scored seven runs. Frustration turned to smiles. The Mets had it all together for a night.  The big inning propelled the Mets to snap a three-game skid with an 8-0 victory over the Nationals before an announced crowd of 33,622 that was much smaller.  In getting swept three games by the Rockies over the weekend, the Mets scored only four combined runs to continue their season-long offensive woes.  Clay Holmes concluded his dominant April by pitching six shutout innings in which he allowed three hits and one walk with six strikeouts. The right-hander’s ERA dipped to 1.75 before Tobias Myers and Craig Kimbrel finished it.  Now the Mets need momentum.…