Trump arrived in the UK late on Tuesday, local time, for an unprecedented second state visit, and will be greeted by Charles on Wednesday for a day of pomp at Windsor Castle, about 40 kilometres west of London.
Earlier on Tuesday, protesters unfurled a massive banner featuring a photograph of Trump and Epstein near Windsor Castle, and later projected several images of the two onto one of the castle's towers.
UK police said in a statement four adults were arrested on suspicion of malicious communications following an "unauthorised projection" at Windsor Castle, which they described as a "public stunt". The four remain in custody.
Democrats in the US House of Representatives on September 8 made public a birthday letter Trump allegedly wrote to Epstein more than 20 years ago, though the White House has denied its authenticity.
The letter was also projected onto the castle, along with pictures of Epstein's victims, news clips about the case and police reports.
The release of the letter has brought renewed attention to an issue that has become a political thorn in the president's side.
Though he has urged his supporters to move on from the topic, appetite for details about Epstein's crimes and who else may have known about them or been involved with him has remained high.
Trump was friends with Epstein before becoming president but had a falling out with the former financier years before his 2019 death in prison.
The birthday letter contained text of a purported dialogue between Trump and Epstein in which Trump calls him a "pal" and says, "May every day be another wonderful secret".
The text sits within a crude sketch of the silhouette of a naked woman.
