The on-going action, by paramedic members the GMB union, will take the form of a full withdrawal of services for day shifts starting between 6am and 8am.
This latest action is due to begin again on Saturday.
The North West Ambulance Service (NWAS) with whom the GMB members are in dispute, says that although the numbers involved is only a small proportion of its frontline staff, the extended time period of the latest action means there will be delays in reaching some patients.
Paramedics who are not members of GMB and our emergency medical technicians (EMTs) will be working as normal.
Ambulance staff across the region - which includes Lancashire, Cumbria, Cheshire and Merseyside - have been involved in on-going industrial action both this month and in June.
Lisa Ward, Interim Director of Organisational Development for North West Ambulance Service (NWAS), said: “The trust is extremely disappointed with this fourth day of action and is frustrated with the reluctance of GMB to discuss any movement from their current position.
“This is despite the positive discussions which have enabled us to reach an agreement on this matter with our other partner trade unions UNISON and Unite, who represent the majority of our staff.
“We have received formal notification that this action will occur every weekend from 21 July until October.
“I am in no doubt that their actions will effect patients although we will be doing all we can to minimise any impact on the public.
“It is fair to say that the action so far has had little impact on our ability to respond to patients, however, this extended period of action is highly likely to cause some delays, particularly to the less urgent calls.
“We have robust plans to ensure as much cover as possible throughout the region but we urge the public to help us by only calling 999 for life-threatening or potentially life-threatening emergencies.”
For non-emergency health concerns, alternative care and advice can be found though local pharmacies, GP surgeries, walk-in centres or NHS111.