This shows part of a row of gleaming white stupas between the two main halls of the Wat.
The four white stupas were built to commemorate the four Buddhas in this Bhadra Age (there are eventually five Buddhas in the Bhadra Age): Kakusandha Buddha, Konegamana Buddha, Kassapa Buddha and the late Gautama Buddha. The coming one is Shri Aryamaitriya Buddha.
(Click on thumbnail above for larger view.)