The stunt happened on the eve of Trump’s state visit to the UK.
A 60-year-old man from East Sussex, a 36-year-old man from London, a 37-year-old man from Kent and a 50-year-old man from London were arrested on suspicion of malicious communications and remain in police custody, Thames Valley Police said.
Chief Superintendent Felicity Parker said: “We take any unauthorised activity around Windsor Castle extremely seriously.
“Our officers responded swiftly to stop the projection and four people have been arrested.”
Other activists this week unveiled a huge image of Trump with Epstein in the area near the castle.
Trump said Tuesday after arriving in London that he loved being back in the United Kingdom, calling it a “very special place”.
Asked if he had a message for Charles, he said the king was a longtime friend of his and well-respected.
ABC journalist John Lyons, on assignment for the program Four Corners, quizzed Trump on his business dealings while in office, asking if it was “appropriate” for a sitting president to be engaged in so much personal commerce.
