Buckle up. Electric vehicle sales are shifting into high gear.

Cumulative passenger EV sales worldwide are set to hit 4 million this week, according to Bloomberg New Energy Finance. The next million EV mark could be reached slightly later if Tesla Model 3 sales slow down, the firm noted. But even then, the milestone would still be well within sight.

Including electric buses, the 4 million threshold has already been reached. At the end of June, there were more than 3.5 million passenger EVs sold globally and about 421,000 electric buses, bringing the total number of EVs sold to 3.97 million.

Sales were driven in large part by China, which is responsible for around 37 percent of passenger EVs sold around the world since 2011 and around 99 percent of e-buses.

EV sales are accelerating, Bloomberg NEF figures show. It took 17 months to go from the first million to the second million EV sales milestone, and just six months to go from the third million to the fourth million milestone.

Bloomberg NEF projects it will take just over six months to sell the next million EVs, reaching the 5 million milestone in March 2019.

Around 42 percent of upcoming EV sales will be in China, 26 percent will be in Europe and 25 percent will be in North America. If Tesla Model 3 sales continue to accelerate, North America could catch up with Europe "quite quickly," Bloomberg NEF states. But there's no guarantee of that happening.

Tesla has been grappling with Model 3 delays, quality concerns and executive churn, as well as a recently abandoned privatization effort. Tesla CEO Elon Musk said last week that going private would be "more time-consuming and distracting than initially anticipated," at a time when the company urgently needs to ramp its Model 3 production and achieve profitability.

In the second quarter of 2018, EVs as a share of new car sales stood at 4 percent, 2.3 percent and 1.6 percent in China, Europe and North America, respectively. Japan, meanwhile, has seen EV sales slump in recent years.

Several new EV models are slated to come to market before the end of 2018, which should boost global EV purchases. Bloomberg expects the Tesla Model 3 will arrive in Europe in mid-2019, which will likely increase EV sales in the region further.

In China, subsidies have played a key role in driving EV sales. The government expects China's annual new energy vehicle (NEV) output to hit 2 million in 2020, and for NEV sales to make up 20 percent of the overall auto market by 2025, Xinhua reports.