Ad giant WPP saw its share price fall to a four-year low on Thursday after reporting a slump in fourth-quarter revenue.

The company reported Q4 like-for-like revenue declines (excluding pass-through costs growth) of 1.4% in North America (1.7% in Canada), 5.1% in the U.K. and 4.8% in the rest of world, including a 21.2% drop in China. WPP CEO Mark Read admitted in the earnings statement that it’s a “tough market out there.”

North America was bogged down by weak spending from health care and CPG. Globally, travel and leisure, retail and pharma lagged in spending.

WPP’s stock was down 15.55% on Thursday morning with shares trading at US$41.

Overall, WPP creative, PR and specialist agencies experienced full-year revenue declines. Its media investor, GroupM, however, bucked the downward trend with 2.7% like-for-like revenue growth, excluding pass-through costs.

In a statement, Read said weaker client discretionary spending played a part in the fourth-quarter results.

“We did see growth from our top 25 clients of 2.0% and an improving new business performance in the second half of the year with wins from Amazon, Johnson & Johnson, Kimberly-Clark and Unilever reflecting the strength of our integrated offer,” Read said.

The company also cited North American client assignments, which include new clients and expanded scope in the retention of existing clients, for H&R Block, Pizza Hut and eBay.

WPP also announced an agency investment hike for its WPP Open intelligent marketing operating system in an effort to “keep it at the forefront of AI and further deploy it across the business and our clients.”

According to the company, WPP Open is now used by 33,000 people across its agency network, which comprises some 100,000 employees in multiple markets.

Read restructured WPP last year via the creation of VML and Burson agencies and the streamlining its of media buying agency, GroupM.