There has been zero scrimping so far. ..... It is right at 2500 sq ft.
Idaho is a mixture of heating climate zones 5 and 6, depending on where you are. Do you know what your town's design minimum temperature is (which I think means it's lower only 1% of the time)?
I suspect that in specifying a four-ton unit the contractor (a) doesn't know how to do a thorough heat loss calculation for a very well insulated house; (b) knows how, but is unwilling to spend the time it takes to figure in all the details for such a house; or (c) at least suspects that a smaller unit would work, but knows also that he makes more money selling the bigger unit and knows that he won't get callbacks for oversizing a system.
At four tons for just 2500 sqft, he's figuring on 625 sq.ft. per ton of heating load. That's far too low for new, very well insulated construction. I won't give you a better rule of thumb to use, because rules of thumb should not be used for this at all; that's just guessing. You ought to have someone knowledgeable do a very detailed heat loss calculation, using actual dimensions, details of construction and insulation of all parts of the house, sizes and U values of all windows and doors, and results of blower door test obtained (or expected, if not at that point yet). You could even do this yourself with a spreadsheet; that's what I did.
For comparison, my house is a total of about 4000 sqft, on two levels, built into a hill. Uphill side of the lower level is full foundation wall, while the downhill side is fully framed. The sides are stepped. Framing is double wall (2x6 outer, 2x4 inner), with a 12" insulation cavity; whole-wall R value is close to 40. Attic is R60. Windows are triple-pane casements and fixed glass. Sub-slab insulation is R20, as are foundation walls. House is very tight, with HRV for continuous low-level ventilation. My spreadsheet told me the heat loss at -3 F would be about 22,000 BTU/hr (1.83 tons). The area distributor for Climatemaster specified a five-ton unit, as did two other installers. None did a really detailed calculation, but used canned software and some incorrect assumptions. In operation since 2011, my two-ton unit keeps the house at a nice even 70 F, in just first stage. My best calculation of actual heat loss at zero, based on limited data over a three-day period when the outside temperature swung just a few degrees to either side of zero, is about 19,000 BTU/hr (1.58 tons). That's about 2500 sqft/ton, although that's a number that certainly does not scale by size. A better, but still very very crude approximation would be by square root of living area, so your 2500 sqft might actually lose only 1.25 tons worth of heat, other things being equal, which they certainly are not. But four tons for only 2500 sqft ????
Bottom line, however you heat/cool the house, you ought to have someone competent do a detailed heat loss calculation, so that you really know what you'll need. Be in the driver's seat on this, and don't just accept what the contractor wants to install.