Scotland's First Minister Alex Salmond has called for calm after a parcel bomb "similar in nature" to those sent to Celtic manager Neil Lennon and two high-profile supporters of the club was intercepted in Northern Ireland.
The Police Service of Northern Ireland said it was alerted after the package was found at a postal sorting office in Belfast which handles returned mail from around the UK.
The latest discovery ensures the case remains in the spotlight but Mr Salmond said: "Everyone should remain calm in light of this new discovery and continue to give Strathclyde Police our full support. Let's be quite clear - the individual or individuals responsible for these disgraceful actions will be apprehended and dealt with with the full force of the law."
The item, found on April 12 at the National Return Letter Centre, had been returned undelivered from an address in Scotland. It is understood the parcel did not originate in Northern Ireland and had no links to the region, and was not addressed to a particular individual.
Officers are still looking for whoever was responsible for sending parcel bombs to Lennon, lawyer Paul McBride QC and former MSP Trish Godman, devices which were confirmed by Strathclyde Police as viable and designed to cause serious harm to the recipients.
Strathclyde Police believe the latest package to be identified originally entered the postal system at around the same time as the devices that are already under investigation.
Chief Superintendent Ruaraidh Nicolson, of Strathclyde Police, said: "This latest discovery will become part of the ongoing investigation. We'd like to stress that there is still no intelligence to suggest that these packages pose a threat to the wider public and we would urge people to remain calm.
"The person or people sending these packages are clearly determined to cause fear and alarm to the individuals or organisations concerned. This is an utterly despicable crime, committed by an individual or individuals who are prepared to put people in harm's way simply to attract attention to their callous and cowardly actions.
"We are grateful to Royal Mail for their continued vigilance and co-operation and would stress that workers have been given the appropriate safety advice."
Two of the four parcels were intercepted at Royal Mail sorting offices in the west of Scotland last month, both addressed to Lennon. The first one was found in Saltcoats, Ayrshire, on March 4 and the second was intercepted on March 26 in Kirkintilloch, East Dunbartonshire.