BERLIN — Four attacks in a week — three of them carried out by asylum seekers — have left Germany on edge and Chancellor Angela Merkel’s policies of welcoming refugees under renewed criticism. Anxiety over Germany’s ability to cope with last year’s flood of more than 1 million registered asylum seekers first surged following a series of sexual assaults and robberies in Cologne during New Year celebrations. But in the last seven days, the violence has become even more deadly. The violence followed an attack in the French Riviera by a Tunisian truck driver who plowed his vehicle into a Bastille Day crowd, killing 84 people in Nice. Experts say the attacks are likely to inflame anti-foreigner sentiment in Germany, creating a challenge for Merkel’s government. Merkel could now face increased calls for tighter border security and greater vetting of arrivals, even though the flow of migrants and asylum seekers has slowed drastically, said Florian Otto, an analyst with the risk consultants Verisk Maplecrof.