The Publicis media agency’s latest Advertising Expenditure Forecasts gave a global growth figure that was down against previous reports in June (4.2%) and March (4.4%).
Zenith is now predicting global ad expenditure to reach $558bn (£423bn), with a marginal downgrade for Western Europe, which is set to grow 2%, down from 2.2% forecast in June.
North America, which Zenith said has suffered from declining network US TV ratings, is expected to grow by 3.6% this year, while Western Europe's growth had been "dragged down" by economic uncertainty in the UK, which is currently negotiating its exit from the European Union.
Asia-Pacific and Latin America forecasts also saw marginal downgrades, but there were upgraded growth forecasts for North America and Central & Eastern Europe.
Next year Zenith expects a 4.2% growth in global adspend, boosted by the Winter Olympics in Korea, the football World Cup in Russia, and the mid-term elections in the US.
Meanwhile, the growth in global advertising is being led by social media in-feed ads, online video and other digital formats such as paid content and native advertising, the report added.
Between 2016 and 2019 they will drive 14% annual growth in total display advertising – a category that includes these formats as well as traditional banners. Total display expenditure will rise from $84bn to $126bn over this three-year period, accounting for 64% of all growth in global ad expenditure.
That means by 2019 total display will account for 50.4% of internet advertising expenditure, exceeding 50% for the first time. Most of this growth is coming from social media (which will grow at 20% a year) and online video (growing at 21% a year).
Jonathan Barnard, head of forecasting and director of global intelligence at Zenith, said: "Internet display is coming into its own as a brand-building media, powered by social media and online video.
"But the distinctions between online video and traditional television are being eroded, and the two work together much better than they do separately."