IOTL, Napoleon IV was killed by the Zulus in 1879, while serving in the British Army. He was twenty-three and had been in exile from France since the fall of the Second Empire.
Suppose the Franco-Prussian War never broke out and the Second Empire had never fallen. Napoleon III likely would have died of his kidney disease sometime in the mid-1870s anyway, leaving Napoleon IV as the new Emperor of the French.
What sort of Emperor might Napoleon IV have made? He was obviously brave, but also seemed more than a little reckless. Would he have been France's version of Wilhelm II? Or might he have matured into a decent constitutional monarch?