not much is known about Shakti III,IV and V apart from all of them were tested for research purpose and to demonstrate expertise in controlling and damping the sub kiloton yield.
Shakti III
An experimental boosted fission device that used reactor grade Plutonium for its primary with a yield of 0.3 kt. This test device was used to test only the primary stage. It did not contain any tritium required to boost the fission. This test was designed to study the possibility of using reactor grade plutonium in warheads and also to prove India’s expertise in controlling and damping a nuclear explosion in order to achieve a low (sub-kiloton) yield.
Shakti IV
A 0.5 kt experimental device. The test’s only purpose was to collect data about the explosion process and to study the performance of various bomb components.
Shakti V
A 0.2 kt experimental device that used U-233, an isotope of uranium that is not found in nature but is produced in India’s fast breeder reactors that consume Thorium. This device too was used to collect data.