$4.000 to install a boiler? That sounds like a lot! Geepers, two guys, one day, how much can they get an hour? Unless that includes building a chimney it seems quite high. You have the oil setup now, they won't be installing or wiring pumps or piping out an oil tank or nonstalling baseboard. Gosh, when I was plumbing two of us would do an "electric change out" in a 16 hour day during the winter. The day before we would get all the parts onsite. We'd start at 6AM and would remove the electric baseboard, install hot water basboard, install & plumb the boiler and have the thing up and running before the house froze. Usually 14 hours, sometimes 16. That was 12 yrs ago, but even now two plumbers one day should not get over $1,200. Maybe $3 or $400 for parts. I'd be getting a second opinion!