"We're hoping for as many talks as possible. The urgency is obviously rising," a CWU spokeswoman said.
Although Friday's strike is on a much smaller scale than those held on previous days, one worker on the picket line in Plymouth told the BBC: "We may be a very small cog in a very large wheel, but we are aiming for maximum disruption."
The Plymouth site is England's largest coding centre - which redirects badly-addressed mail.
Royal Mail says it is trying to modernise to compensate for letter volumes dropping by 10% every year as people switch to other forms of communication such as emails and texts.
It has shed 63,000 frontline postal staff in recent years, and says it needs to cut more jobs as part of continuing modernisation plans.
The CWU agrees that job cuts are necessary, but disagrees over their extent, and over the future pay and working conditions of the workers that remain.
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