This book contains biographies, about 70 pages each, of James Boswell, historian Edward Gibbon, novelist and clergyman Laurence Sterne and politician John Wilkes, all of whom, each in their own way, made a distinct mark on late 18th century UK.
The Boswell biography in Four Portraits is, as most biographies of Boswell published prior to 1950, somewhat faulty due to a lack of sources. Quennell seems to draw heavily on Vulliamy's 1932 biography for at least some of his information, which doesn't help much. James Boswell's relationship with his father is (correctly) described as somewhat difficult, but then Quennell states that Boswells first visit to London in 1760 had been permitted by his father, and that his 1762-3 stay in the city was the beginning of his grand tour, also agreed to by his father. Alexander Boswell actually did not permit James' 1760 stay in the city, and called on his Ayrshire neighbour Lord Eglinton to check up on him - he then allowed James to stay a bit with the Lord. Alexander Boswell also only grudingly accepted that James stayed in London for a bit in 1762-3 to try to get a commission in the guards, and it was only in the spring of 1763 that any mention of a Grand Tour came up. Another typical misconception is Quennell's belief that Boswell obtained leave to dedicate The Cub at Newmarket to the Duke of York, which was not the case. James is said, by Quennell, to have been born at Auchinleck, while most scholars today agree that he was born in the Edinburgh home of Alexander Boswell, who had not yet inherited Auchinleck in 1740.
Most of the biography of Boswell deals with his relationship with Dr. Johnson, and in that it seems to be mostly correct - probably because it draws mainly on Boswell's own Life of Johnson, published in 1791.
The book is interesting, however, and the biographies of Gibbon, Sterne and John Wilkes seem much less inaccurate than the one about Boswell. Boswell became a good friend of Wilkes' and may also have met Sterne in London in 1760.