ROME (AP) — More than 4,000 would-be refugees were rescued at sea Thursday in one of the busiest days of the Mediterranean migrant crisis, and at least 20 died trying to reach Europe as Libyan-based smugglers took advantage of calmer seas to send desperate migrants north.The death toll was likely to grow far higher, however, as the Libyan coast guard also reported two overturned boats between the coastal cities of Sabratha and Zwara.