By Anna Wise, PA Business Reporter & Lawrence Matheson Advertising boss Sir Martin Sorrell has warned that businesses will continue to be careful with their advertising spending until 2024 due to “unpredictable times”. His digital marketing company, S4 Capital, expects sales to have dropped over 2023, matching its earlier lowered expectations. Sir Martin, who founded and led the world’s largest advertising and marketing firm WPP before launching S4 Capital in 2018, told shareholders that clients had been cautious with their spending over the past year. He said: “After four years of very strong…