Both sides had begun the night with perfect records in Group D, but it was the Oranje who took control of their destiny as they overcame their hosts in Bucharest.
Lens opened the scoring with a stunning header from outside the area - the first goal Romania had conceded this campaign.
Holland then had the game in their keeping by half-time as Bruno Martins Indi added a second before Rafael van der Vaart converted a penalty moments after Ciprian Marica pulled one back.
Robin van Persie sealed the win late on, and while Lens admitted the scoreline flatted the Oranje the result left them in full control in the race to Brazil.
"The most important thing is that we won the match 4-1," he was quoted as saying in De Telegraaf.
"We are not there (qualifying for the World Cup) just yet. However, we have taken a big step toward qualification."
The PSV Eindhoven forward was pleased with his opener, adding: "The ball landed perfectly on my head and I could head it home nicely.
"It was very nice to score the opener as Romania were putting the pressure on us."
Boss Louis van Gaal admitted Lens' goal had eased the pressure on his side after they had been forced on to the back foot early by the Romanians.
"This is a great achievement," he told SBS6. "We did not start so well.
"But luck was on our side. Then we got better in the game and we saw it coming. They were incredible goals. Lens' [goal] for example, who never [scores] headers.
"I thought in the first quarter we were restless, which I had not expected. But we had problems getting the ball forward.
"Romania are organised, so maybe that's the excuse. In addition, the stadium was beautiful but the pitch less so."