CREATE TABLE table_3891 ("Model - Computer name" TEXT, "CPU type" TEXT, "No. of processors" TEXT, "Period of operation" TEXT, "Maximum peak performance  (  Teraflops  ) " TEXT, "Use" TEXT)

Name the number of processors for nec sx-4 vector processor
SELECT "No. of processors" FROM table_3891 WHERE "CPU type" = 'NEC SX-4 vector processor'
SELECT "No. of processors" FROM "table_3891" WHERE "CPU type" = 'NEC SX-4 vector processor'