Quote Quoting sandor
1. What is the process that is needed to evict just 1 roommate?
The most likely avenue would be for the landlord to commence an eviction action against that roommate.

Quote Quoting sandor
2. Is it the landlord's responsibility to start the process or do the roommates need to initiate this?
The roommate's contract is with the landlord, so you probably need your landlord to initiate the eviction.

Quote Quoting sandor
3. What is our liability if the property is damaged by the 4th roommate?
The other roommates are jointly liable to the landlord, to compensate the landlord for any damage.

Quote Quoting sandor
4. If we ask him to leave and he does so voluntarily are we legally free to get another roommate?
If your lease and/or landlord permits.

Quote Quoting sandor
5. Are there any other issues that we may be overlooking?
Quite possibly; a local lawyer can advise you.