Will all due respect for your friend's methods, 1400 gallons a year does
NOT imply a 168,000 BTU/hr boiler (or even
HALF that.) Even assuming a cool part of SE PA (say Allentown or Scranton) your
99% outside design temp is about +7-8 F, and you run about
5800 annual heating degree days (base 65F) .
That's 1400 gallons over 5800 HDD, or 0.24 gallons per HDD, which is 0.01 gallons per heating degree-hour.
There are 138,000BTU source fuel BTUs in a gallon, but at 85% efficiency only (138,000 x 0.85=) 117,300 BTU of that gallon end up in the heating system (the rest went up the flue. That means you're only taking( 0.01 gallon/degree-hour x 117,300 BTU/gallon=) 1173 BTU per degree-hour.
Assuming a 65F outdoor temp as the balance point between heating/cooling loads (it's close enough, which is why they use base 65F as the standard for HDD), and an even cooler than Scranton 99% design temp of +5F that's only a 60F heating-degree delta, and an implied heat load of ( 60F x 1173 BTU/F-hr =) 70,380 BTU/hr.
That's a very credible number for a 4000' house that has at least some insulation, and storm windows over leaky antique double hungs.
And that's most likely an upper bound, since boilers oversized by more than 1.7x don't actually hit their AFUE numbers due to the lower duty cycle, and higher standby loss. And with four zones the boiler is cycling on/off on zone calls a LOT, and is probably way-oversized for any single zone's radiation, making it cycle even when there is a continuous call for heat from a single zone. (You can measure that- time the burns!) If you're also heating hot water with oil, your true heat load is probably closer to 60K.
If you have oil fill-up dates & quantities during the heating season we can refine that a bit, but it won't change the fundamental issue that the existing boiler is more than 2x overized for the current load, and even more oversized for a home undergoing weatherization.
A right-sized boiler plumbed correctly for the radiation (so as not to suffer condensing from return water that is too cool) will have longer, more efficient burns, and a higher duty cycle, meeting it's AFUE numbers. If you are continuing to tighten up the place don't oversize over the 70K number by even 10%, or you'll end up with the same oversized over-cycling & low duty-cycle condition that you're in now, which may have contributed to it's early demise (though I's suspect condensing return water temps more than short-cycling as the proximate cause of a leak.)
The very smallest oil-fired boilers out there are in the 60-70K range, and that's what you should be looking at. If you've been heating hot water with an embedded coil in the boiler, an indirect-fired hot water heater operated as a 5th zone in "priority" mode would be the right solution, since the smaller boiler doesn't have the capacity to support simultaneous hot-water + space heating loads, but is enough burner to support an "endless shower" at 2.5 gpm at SE PA incoming water temps.
While it's unlikely to be a total solution for a 4000' house, ductless air source heat pumps (mini-splits) can heat a large zones for about half the cost of heating with oil, and have the additional benefit of high-efficiency air conditioning as well. A 1.5-ton mini-split can deliver ~22,000 BTU/hr at +5F, and costs about $4-4.5K installed (half that if you do most of it as a DIY, and pay a tech with the all the right tools & experience for the final refrigerant charging & test.) Even though that's less than half your whole-house load @ +5F, it has more capacity and even higher efficiency at 30F, covering most or all of your heating load at your AVERAGE mid-winter heat load. In a home with an open floor plan it would cut your oil use by more than half, possibly as much as 3/4 and reduce your overall heating bill by more than a third. (The details will vary.) A room-by-room, zone-by-zone heat load calc would be necessary to size ductless systems correctly, but most of the time if right-sized they pay for themselves in three heating seasons or less on heating-cost savings.