A comparable nation is the Russian Empire, which in the time period 1796-1918 had 2 Assassinations of Tsars, Tsar Paul I and Tsar Alexander II. In addition there were failed assassination attempts on Grand Duke Konstantin Pavlovich and on Nicholas II when he was the crown prince. Nicholas II was also executed by the Bolsheviks, but I am not sure if that counts as a assassination. So in the time period 1789-1918, the US and the Russian Empire had about the same prevalence of assassinations and assassination attempts.
Well when religion played a big part in the British monarchy, several kings, queens and other royal folk were killed (e.g. Mary I, James I, Charles I, Lady Jane Grey, Edward V & Duke of Shrewsbury aka 'The Princes in The Tower', several of Henry VIII's wives etc)
Also much later Queen Victoria survived 3 assassination attempts in June 1840, May 1842 & March 1882.
However if we compared assassinated presidents to assassinated UK prime minsters the US would win. There has only been 1 assassinated UK PM, Spencer Perceval (a Tory not a Conservative) who was in office from 1809-1812.
According to Denis Gray, On the morning of the 11th May 1812, a man by the name of John Bellingham waited in the House of Commons lobby for Perceval. Bellingham shot Perceval to whom it's thought he said 'I am murdered. I am murdered' and died. But why would Bellingham want to shoot the PM?
Well according to Gray, as an adolescent Bellingham had been sent to be a merchant's clerk in Arkhangelsk, Russia where he'd been imprisoned for accumulating debt. Bellingham grew bitter, not with the Russian government but with 1st Earl Granville, The British ambassador to Russia & in turn the UK in general.
Upon his release, Bellingham tried to avenge himself and get the government to give him compensation. His continued failure further disillusioned & embittered him. He began to practice with his pistols ready to give Perceval his comeuppance.
Sources:
Gray. D (1963): "Spencer Perceval: The Evangelical Prime Minister, 1762-1812"
BBC News (2009): "The MP whose ancestor killed the prime minister"
BBC Parliament (2012): "The Murder of Mr Perceval"
Who are the 4?
I can think of McKinley, Kennedy and Lincoln