The force said: “While formal identification is yet to take place, we believe we know the identity of the deceased but are awaiting confirmation.
“His next of kin have been informed and our thoughts are with them at this very sad time.”
A fifth person was confirmed dead by police in Sheffield Sunday evening.
Emergency services were called to a man in his 20s coming into breathing difficulties at an unused quarry in Crookes Valley Park, Sheffield and pulled the man’s body out of the lake just before midnight on Sunday.
On Monday, the Met Office issued its first-ever extreme heat warning, which has been applied to South Wales, West Midland, southwest England and further east into Hampshire as far as the Isle of White.
The new weather warning is in place until 23:59 Thursday, with temperatures expected to drop on Friday.
It was also the hottest day ever in Northern Ireland on Saturday, with 31.2C (88.16F) recorded in Ballwatticock in Country Down.
The hot weather is set to continue this week, with the Met Office forecasting 32C (89.6F) in some parts of the country.
The Met Office is working with Public Health England (PHE) to ensure that people stay safe in the hot conditions, urging the public to stay hydrated, wear sunscreen and assist anyone who is more vulnerable to the heat.
The extended hot spell of weather is expected to last for much of the week ahead, with temperatures in some parts of the country due to hit the high 20s.