The bodies have been passed to a vet for post mortem examinations
The RSPCA is investigating whether some farmers have taken a proposed cull of badgers into their own hands.
Four animals were found dead in a field near Crymych, in Pembrokeshire on 18 April.
Tests are being carried out on their bodies as there were no visible wounds on the badgers, the charity said.
The Welsh Assembly Government announced a cull of badgers earlier this month as part of a pilot project to stamp out tuberculosis in cattle.
However the location of the area and details of the cull have yet to be decided.
RSPCA inspector Gemma Black said: "These badgers died in suspicious circumstances, but we need the public to help us pursue this matter.
"It may be that some farmers have decided to kill these badgers following the Welsh Assembly Government's decision to cull badgers as part of the campaign to eradicate TB in cattle.
"We would like to remind everyone that the minister for rural affairs has said that this cull will be licensed and monitored by assembly government and that any other form of badger killing will be illegal."
A member of the public alerted the RSPCA to the incident on 18 April.
Dai Davies, president of farmers' union NFU Cymru, said: "We don't condone this behaviour.
"Badgers are still a protected species and if any behaviour like this is seen, people should call the police," Mr Davies added.
"As the minister herself said the other day, there is a proper procedure supposed to be in place for this and we welcome that."
An assembly government spokesperson confirmed the badgers had been passed to them for a post-mortem under the Wildlife Incident Investigation Scheme.
"When the Rural Affairs Minister made the announcement about the TB eradication programme it was made clear that the badger remains a protected species in Wales and the conditions of the Badger Act are firmly in force," the spokesperson said.
"Illegal action will not be tolerated."
Dyfed Powys Police said they were aware of the incident, but would not become involved in the investigation unless the RSPCA requested
Anyone who may have information about the incident may contact the RSPCA on 0300 1234 999