Rescuers looking for the group are focusing on two areas identified in the wreckage by sniffer dogs, fire chiefs said.
A potential criminal investigation could be launched as police detectives and Health and Safety Executive investigators begin to examine the cause of the blast.
The four still missing, three men and a woman, are believed to have been in the building near the seat of the explosion that ripped through the wood flour mill at around 9.10am yesterday in the village of Bosley, Cheshire.
No deaths have been confirmed as rescuers continue to search but hopes are fading for finding anyone alive.
Paul Hancock, chief fire officer for Cheshire Fire and Rescue Service (CFRS), said specialist rescue teams were still carrying out a painstaking search of the site, a smoking mass of twisted metal and rubble.
The Sunday service at St Mary's Church in Bosley is expected to attract villagers to pay their respects.
Cheshire Police said they were working closely with the families of the four missing but were not yet releasing names. All four are believed to be local, with one from the village of Bosley.
A total of 35 casualties were assessed at the scene with four people taken to hospital.
A 29-year-old woman was airlifted to Queen Elizabeth Hospital, Birmingham, after she suffered serious burns and blast injuries to her head, face, arms and chest.
Another two people were taken to a specialist trauma unit at Royal Stoke University Hospital, while a fourth was taken to Whiston Hospital, Merseyside.
Up to 80 firefighters from Cheshire, Derbyshire, Greater Manchester and Staffordshire fire and rescue services attended and were supported by colleagues from the West Midlands and North West ambulance services, together with Cheshire Police.
The mill, which made linoleum products out of wood ground down to a powder or flour, has been producing wood flour since 1930 and is the sole manufacturer in the UK.
A nearby Methodist church was damaged in the explosion.
