Hywel Dda University Health Board, the Food Standards Agency and Ceredigion County Council are also looking into the unpasteurised or raw cow’s milk produced at the farm near New Quay.
Visitors to the Royal Welsh Show in Builth Wells may have sampled or bought the milk, which was available on 26 July.
The four cases all consumed or bought the milk at Aberystwyth Farmer’s Market after 1 June.
Public Health Wales is notified of between 3,000 and 4,000 cases of Campylobacter every year.
Dr Brendan Mason, consultant in communicable disease control for Public Health Wales, said: “If you have consumed Penlan y Môr unpasteurised milk any time since 1 June, including at the Royal Welsh Show on Wednesday 26 July, and you are experiencing symptoms of Campylobacter infection, we want to hear from you.
“It is a very unpleasant infection, and more serious complications can occur. As in any diarrhoeal illness, washing hands after using the toilet and before preparing or eating food is very important and anyone worried about their symptoms should seek medical advice.”