When I was on the DC 6 we started inboard,outboard (same side)inboard,outboard (other side). An inboard was always started first because that is where the hyd pumps are. The order could be 3,4,2,1 or 2,1,3,4 at the captains discretion. A factor was whether the captain started all engines or whether the captain started the right wing engines and the copilot started the left side engines (the better way IMO). In the later case the convention at my company was 3,4,2,1.