The Board of Appeals on Wednesday will hold public hearings on a four-unit condo project and a two-unit affordable housing project. A home at 42 Sylvan St. would be converted to four, three-bedroom town-house-style condominiums. Permission to allow four parking spaces in the front yard is also sought. Separately, the Melrose Affordable Housing Corp. has applied for a special permit to convert a single-family home at 165 Trenton St. into two units of affordable housing. The hearings will be held as part of the board’s meeting, scheduled for 7:45 p.m., in the aldermen’s chamber at City Hall.