'Dharma' purusharth is first to other three purusharthas. It's place of priority indicates that:
1
Artha, Kāma and Mokșha are lower purusharthas.
2
The three become purusharthas in connection and reference with "Dharma".
3
Purusharthas are not related to each other.
4
Moksha purushartha merges all other three in it.