The result means that Team GB have now equalled their best ever performance in the pool at the Olympics, matching the record of four golds, two silver and a bronze from the London 1908 Games.
The medal also marks the second relay gold following the victory of the men's 4x200m relay team - of which Guy was also a key part - on Wednesday.
Peaty now boasts a total of three golds and one silver from Tokyo and Rio 2016, while Guy now has two golds and two silvers - all from relay events.
Hopkin was given the task of bringing home the gold for GB in the fourth leg after Dawson, Peaty and Guy had built up a lead of around 0.6 seconds over the Chinese, who claimed the silver.
The USA were the only team to place a male on the fourth leg, giving rise to the unusual spectacle of superstar Caeleb Dressel competing against seven females in the freestyle, but even he was unable to make up an eight-second deficit on the triumphant Brits.
Team GB have three more opportunities to medal tomorrow on the final day of the swimming programme, with Ben Proud featuring in the 50m freestyle final, Daniel Jervis in action in the 1500m freestyle final and a men's team including Guy taking on the 4x100m medley relay.