The firm collects data from thousands of smartphone owners via an app.
Of the four UK networks, EE was found to have the best 4G connectivity, while Three had the worst.
However, Three also had the fastest download speeds when it was connected to 4G, the researchers found.
Three was the last operator to roll out 4G in December 2013 and told the BBC it was continuing to extend its 4G network every day.
"Almost everyone now uses a mobile phone service and it's not good enough that the UK is lagging behind so many countries with our 4G network coverage," said Richard Lloyd, executive director of Which.
The State of the Mobile Network report also noted that O2 had the highest 3G and 4G latency - that's the amount of time data takes to travel along a network, measured in milliseconds.
The firms have committed to investing in mobile networks. Last year, EE unveiled a Â£1.5bn three-year network investment plan that included the expansion of 4G and the commitment to reach 99% of the population with "superfast 4G".
However, mobile phone providers have faced planning issues when creating the necessary infrastructure, said analyst Kester Mann from CCS Insight.
"Network operators will maintain that they are investing millions of pounds each day in network coverage and capacity to satisfy the insatiable demand for mobile data services among UK consumers," he said.
"They will also cite tough planning restrictions to allow taller masts to provide wider coverage, a challenge recognised in George Osborne's recent Budget.
"These findings show that, although the UK is a leading market in the uptake of 4G services, there is still plenty of work to be done in network roll-out."
Smartphone users in the UK can only access 4G's data service about half of the time on average, according to a new report by OpenSignal and consumer watchdog Which.