Officers are investigating the fatalities after post mortem examinations on two of the birds revealed illegal poisoning.
A cause of death for another bird has not yet been established but examinations remain ongoing.
In January, a red kite was discovered in Kirkpatrick Durham and this was followed by the discovery of a buzzard in Mossdale in March.
A further two poisoned red kites were discovered in April, one again found at a separate location in Kirkpatrick Durham and the other being discovered near to the Old Bridge of Urr.
The remaining red kite, the cause of death for which is yet to be established, was discovered at the end of April at Gelston.
The officer leading the investigation, specialist wildlife crime officer constable Alan Steel said: “An investigation has been launched and remains ongoing in respect of the deaths of all four birds and we have been working very closely with SAC Consulting (SRUC – Scotland’s Rural Colleges) and Science and Advice for Scottish Agriculture (SASA) to establish as much information as possible relative to the deaths.
“What we have established is that illegal pesticides have been used to kill four of the birds.
“The pesticides identified have been banned in the UK for many years but despite this there would still appear to be those who leave out poisoned bait, whether that is to target crows, foxes, raptors or other wildlife.
“The use of such poisoned bait is illegal and totally unacceptable and those responsible should understand that their unlawful activities not only serve to have devastating consequences on their intended targets but also on various other forms of wildlife.”
Searches have taken place in the vicinity of where the animals were found and more and expected for the future.
Police are also working with land owners and farmers as well as the RSPB.
Anyone with any information should contact police on 101 or can anonymously call Crimestoppers on 0800 555 111.