The former world champions, Germany, have secured top spot in their qualifying Group J, with two fixtures left to play.
Two second half goals from Timo Werner and one each from Kai Havertz and Jamal Musiala lifted Germany to 21 points.
Romania are in second place on 13 points with two games remaining.
All group winners qualify automatically for the tournament in Qatar.
“From the result it was perfect. We got what we wanted, qualified as quickly as possible and we now have five wins in a row,” coach Hans Flick, who succeeded Joachim Low earlier this year, said.