roachtest: acceptance test cluster size should be 4

For some reason I thought the local cluster size for the legacy
acceptance tests was 3. It is actually 4, so we should use 4 for the
roachtest-based acceptance tests.

Release note: None

pkg/cmd/roachtest/acceptance.go

@@ -20,7 +20,7 @@ import "context"
 func registerAcceptance(r *registry) {
 	spec := testSpec{
 		Name:  "acceptance",
-		Nodes: nodes(3),
+		Nodes: nodes(4),
 	}
 
 	testCases := []struct {