The Barnstable Conservation Commission voted Thursday to continue for a fourth time its review of a polarizing proposal to build a pier, ramp and dock in a strictly regulated shellfish overlay district in Cotuit Bay. Donald MacKinnon, a trustee of Oyster Place Realty Trust, has applied for a special permit to remove 3,000 square feet of an existing historic, solid fill wharf first licensed in 1874. In its place, MacKinnon proposes building low vinyl bulkheads; relocating existing rock[ installing a replacement pier, ramp and floats; and building a seasonal town office building on a 15-by-30-foot deck at 33 Oyster Place Road and 910 Main St.