The 5 are the elites of the village, these 4 are chosen for their areas of expertize, and are given access to some forbidden and powerful jutsu, mot elite are either of the Sanin or Jounin rank, but are chosen by the Kage so can be any rank.

the 4 act as the replacement for the ANBU in the village, since the ANBU were found corrupt and unjust, the current Ho Kage eliminated them, and selected the 4 to defend the village.

Each one of the 4 are the best in their selected field:
Ninjutsu
Genjutsu
Taijutsu
Medical