Prosecutors in NY haven't identified what specifically they're probing, although they said in a court filing that "the crimes being investigated involve acts of concealment by Cohen".
"I never retained him in the traditional sense as retaining a lawyer, I never received an invoice from Michael, I never paid legal fees to Michael, but I have occasionally had brief discussions with him about legal questions about which I wanted his input and perspective".
Judge Kimba Wood ordered Cohen to attend and provide information about his clients as she weighs an emergency action by Cohen's attorneys to stop prosecutors from reviewing more than a dozen electronic devices and documents seized during a Federal Bureau of Investigation raid of Cohen's office, home and hotel room last week.
Michael Cohen outside court in NY on April 16.
The three other clients, according to the letter, are Trump, Broidy and a third client whom Cohen initially did not want to be identified.
Ms Daniels, whose real name is Stephanie Clifford, is entangled in a separate civil battle with Michael Cohen, the president's lawyer, over a $130,000 payment made to her as part of a 2016 agreement to stop her from discussing a sexual encounter she says she had with Mr Trump a decade earlier.
Trump has also brought in his own lawyers from Spears and Imes to protect his interests as one of Cohen's clients.
Michael Cohen, the Trump lawyer whose office was raided by the Federal Bureau of Investigation last week, had a pretty exclusive client list.
Cohen's legal team has been trying to seal evidence taken in the raids.
Cohen, who is under criminal investigation for possible bank fraud and campaign finance violations, wants the material to be reviewed before federal prosecutors have a chance to scrutinize it.
Among his dealings, Mr Cohen arranged a $1.6-million payment to secure the silence of a former Playboy model who said she became pregnant by Mr Broidy, a person familiar with the matter said on Friday. "Upon information and belief, the unnamed legal client's matters are responsive to any matter covered by. the search warrants".
The news emerged after a federal judge in NY ordered the name released from documents seized in Cohen's office last week, multiple sources reported.
It is unusual but not unprecedented for criminal investigators to seize documents from a lawyer, and there is a policy in place created to shield information covered by the attorney-client privilege.
Communications between lawyers and the people they represent are typically kept confidential under the doctrine of attorney-client privilege.
The searching of any attorney's office by the government is highly unusual and typically triggers the appointment of a "taint team" to check seized material for attorney-client privilege. Trump will be represented by his own lawyer, Joanna Hendon, who has requested that the president be given a chance to review the seized materials, which includes more than a dozen electronic devices.