This is a group of four large stupas at Wat Pho, each standing 42 metres high. The first one was built by King Rama I whereby the remnants of the golden Buddha from Ayutthaya, that was melted down and stolen by the Burmese c1767, was to be kept. King Rama III built two more for himself and his father, Rama II. the final stupa was built by Rama IV.
From what I have read these were the first four kings of the Chakri Dynasty who all happened to be alive at the same time although King Mongkut, who eventually became Rama IV, would only have been five years old when King Rama I died. It is believed that King Mongkut ordered all four pagodas to be enclosed and grounds around them to be walled so that no more stupas could be built. According to literature King Mongkul believed that no more stupas would be required.