It meant McIlroy matched the 68 of playing partner Willett, who lies second in the R2D standings and has the best chance of denying the former world No 1's attempt to lift the near $2m bonuses that will go to the season's top player.
South African Branden Grace is also four under, keeping his slim hopes of winning the money list title alive.
Ian Poulter made the ideal start in his bid for a first win in three years as defending champion Henrik Stenson surprisingly found himself at the wrong end of the leaderboard.
A flawless 66 gave Poulter a share of the lead on six under par alongside Ryder Cup team-mate Martin Kaymer, England's Andy Sullivan and Australian Marcus Fraser at Jumeirah Golf Estates.
But world No 6 Stenson slumped to an opening 77, his worst score in 21 rounds in this event, to lie joint last on five over par and faced with a monumental task to claim a hat-trick of wins in the European Tour's season finale.
McIlroy had the Race to Dubai wrapped up with an event to spare last year after four victories, including major titles in the Open and US PGA Championship at Valhalla, but has a lead of just 1,613 points over Willett after an injury-hit campaign.
The 26-year-old admitted to playing some "ugly golf" on the back nine and caused more damage than intended when chipping on the 17th green, adding: "I didn't catch it heavy but I took a little more of a divot than I wanted and came up a bit short.
"But it's nice to get around in 68. I felt like it was a little bit of a struggle at times out there, but to finish the way I did makes it feel a lot better. It never really looked like a four on 18. I hit into the bunker off the tee, hit a good lay-up shot but terrible third shot.
"It was one of those ones where you just had to get it out and it could trundle its way toward the hole. I knew I could get it close (but was) pleased to see it drop in."
Sheffielder Willett admitted he had visions of missing his birdie putt from three feet after McIlroy's bunker shot, but calmly holed to complete a bogey-free round and keep the pressure on the world No 3.
"It's quite a short week after flying in from China, trying to get your body back on the right time," Willett said. "I had a good couple of days' practice but still would like one more. The two of us didn't play our best but four under will put you right up there."
Poulter started the week a lowly 37th in the Race to Dubai - his lowest ever finish is 31st as a rookie in 2000 - and admitted he needed to get back to winning ways after last tasting victory in the HSBC Champions in 2012.
"There's a lot at stake," said the 39-year-old, who started the week by having a cortisone injection in a niggling foot injury. "I think more of it is probably pride than anything else.
"Dropping outside the top 50 (in the world) isn't very nice. Being down the list on the Order of Merit is not very nice. And putting some points on the board for the Ryder Cup would definitely be very helpful."
After Fraser had set the early target in the fifth group out, playing partners Kaymer and Sullivan carded matching rounds of 66, with Sullivan's birdie on the 18th drawing a massive cheer from his personal fan club.
Sullivan, who is the only three-time winner on the European Tour this season, was being roared on by 30 members of his golf club in Nuneaton and said: "Every putt that went in they were really cheering and it's a big plus.
"They've been to Wentworth, Woburn and Portugal this year and have been planning this trip since the middle of the year, dying to get out of the cold at home."