Handling long term mothered tenants

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I am new to the forum and this will be my first building. The building is in excellent shape presently owned by an 80 year old woman who has "mothered" her long term (ten years or more) tenants. The rents are presently $200 under market and each of the four apartments are on a month to month lease. I can buy this building for a very good price well below market and my loan terms will allow me to turn a profit even at the present rents. However my return would be about 4%and that is a hobby and not an investment in my eyes.
Question- how would you go about raising the rents and income on this building? There is a new four car garage with EDO, the tenants use their own washer and dryers but the owner's water and electricity. No AC in the building. I would like to keep the tenants.