A new 4000 sq ft house shouldn't need a 310. unless it was poorly built. And 250 m ay also be over sized.

For 50 bucks, you can do your own load calc, and know what size you need, don't fudge the info, enter it as the house is really built, and use the actual design temp fo your area, not a safe colder temp. http://hvaccomputer.com/talkref.asp

No reason to guess.