Holly Aird (Dr. Wharton) was expecting her second child during season 4 which is why in season 4 her character is nearly always seen standing behind something (or someone in walking shoots) or wearing large coats and lab coats.
In "Cold Fusion", when Spence examines the signing-in book to the building in which the CCU are housed, two of the names shown in the book are Simon Le Bon (famous as a member of Duran Duran) and Bob Carolgees (famous for his comedy act with Spit the Dog). The latter is shown as visiting Superted at the Great Big Gay Bear Company.
The trivia item below may give away important plot points.
When Claire Goose left Casualty (1986) she originally asked to be killed off, but the producers refused. When she began working on this series, the same producers also began working on the show. When she decided to leave at the end of S4 she told the producers they would have to kill her off now.