You know, it might be something as simple as the fan speed not being properly setup. Is it a model that automatically adjusts the fan speed or is it a fixed speed setup (one speed for AC, another speed for heat)?
Did they do a manual J calc to figure out the heat load before changing the size of the unit? There are a lot of factors, but the 4 ton seems like it might be too big. I live in a similar climate, larger house (~3100 sqft), and probably older house (1968). Anyway, I have a 3 ton AC and 24 kW electric furnace. It is a 2-storey house w/ finished basement. There are two zones (controlled by motorized dampers, only single air handler). Upper floor on one zone, the main floor and basement on the other zone. Like I said, there are a lot of other factors too, so the only way to know what size of unit to use is to do a manual J.
If they did do a manual J and figured that you did need 4 tons, then they should have done an additional calc to make sure that your ducts were sized to support a 4 ton unit.
In terms of the electric furnace, the larger (or smaller) unit won't save you any money. With electric, heat is heat, so a larger unit will run less but will also use more power when it does run. You might save a tiny amount with the blower running less, but the power that the blower uses is nothing compared to what the actual heaters pull. I am surprised that you didn't go with a heat pump or gas furnace. The only reason I am still on the electric furnace is that the AC unit is still good. Once it dies, I'll go for the heat pump. We live in pretty mild climates, so a heat pump does pretty well most of the time. As you know, the electric furnace is very expensive to run.
Some other things to look at:
- check your filter(s). Even if clean, some types cause a large pressure drop. You might want to try a cheapie filter (the ones that are under $1) to see if that changes the flow
- besides the supply size, the return also has to be large enough to support the flow. Look at all of the returns (grilles) and check their size. Make sure that none are blocked. Sometimes, they will also install filters at large returns. Maybe there is a clogged filter in the system that you never knew about?
- get a tech to check the blower speed, static pressure, etc. This will tell you if the air handler is setup properly and if the ducts are sized correctly.