The wounded man was found in an alley near the 200 block of Craig Street shortly after 7 a.m. PT, after someone called 911.
Mounties say the wounded man had life-threatening injuries, and paramedics worked on him in the alleyway before taking him to hospital.
Nanaimo RCMP Const. Gary O’Brien said he should survive.
"The victim is expected to recover but will remain in hospital for several days," O'Brien said.
The four teenagers who were arrested were inside a nearby home.
O'Brien said police were applying for a search warrant in order to search an apartment unit where the fight or attack took place.
Google Map: 200 block Craig St., Nanaimo