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."

Norrie and Claire Aitchison said that Liam's killers should have been kept in custody before and during their trial.
The Aitchisons said they will campaign to have the rules changed.
On Monday, Johnathan MacKinnon and Stefan Millar, both 22, were convicted of murdering Liam.
The two men had lived on Lewis.
Following their arrests in December 2011, Millar was allowed bail and moved to Inverness. MacKinnon was bailed in February 2012.
A jury at the High Court in Glasgow convicted the two men of murder.
The court had heard that Liam was beaten, stabbed and stripped his of clothing during an attack on 23 November 2011.
The body of the 16-year-old, from South Uist, was later found in a derelict property in Steinish on Lewis.
Following the conviction of the two men, Mr and Mrs Aitchison have spoken out against the right of murder suspects to bail.
They said the knowledge that MacKinnon and Millar were out on bail had caused their family distress.
Mr Aitchison said his youngest son, Steven, eight, had shared a room with Liam when he stayed at home and was terrified after his older brother was murdered.
He said: "The wee fellow wouldn't sleep and thought he was next - we had to move him from the downstairs room to the upstairs room because the downstairs room was by the front door.
"He had to watch us lock the door and if I wasn't in he had to wait until I was before he would go to sleep"
"We want to head up a campaign to keep anyone that is charged with murder on remand until proven innocent or guilty."
The Aitchisons said that they also hope to set up a memorial garden for Liam.
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.
[NEXT_CONCEPT]
The father and step-mother of murdered Western Isles teenager Liam Aitchison have called for bail to be refused to people who are charged with murder.