Noise complaint by tenants in multifamily

[Kajal Aggarwal]

Hi, I know this is a common problem in multifamily but I honestly don't know how to handle this since I'm not at the property myself and the situation escalated where the police has been involved. 

Unit 1 is vacant and Tenants 2 and 3 are complaining about the noise of Tenant 4. Tenants 2 and 3 are both good, long term tenants who pay their rent on time and take care of the property. Tenant 4 has been here since a year. Tenant 2 has spoken with Tenant 4 multiple times about the problem as they share a wall upstairs. Tenant 4 are college students and they have guests until late night and that is the source of the noise. Tenant 3 is downstairs and affected as Tenant 4 guests go up and down the stairs. Tenant 4 is not ignoring the complaints and try to pay more attention for 2 weeks and the pattern repeats. I have reminded them about quiet hours portion of the lease. 

We get complaints from tenants 2 and 3 atleast once a month and last night they called police. This morning tenant 4 sent a mail about this and says they have not been loud and are being targeted for any small thing. Later in the day tenant 2 also sent a mail asking for a resolution.

I don't see how I can take action against one tenant based just on the word of other tenants, even if I believe the complaint is most likely valid. I asked for a meeting with all 3 tenants to resolve the issue but tenants 2 and 3 refuse to attend as they don't want to be confrontational and that they had bad experience talking to them before. 

Tenant 4 lease ends in July. Should I still talk to tenant 4 and give them a notice or should I just throw up my hands and tell them to work it out? Any ideas?