2400' would work around here for a 4 ton in all but dry soil.
There would be little consequence or benefit to adding 300'. The 2400' is likely a little oversized for your area already and probably based on the fact that your guy gets pipe in 600' rolls. If you must tinker with his design, you may as well go for the whole 600' loop, but again, this is completely arbitrary.
To properly dexign the loop field we would need to know the manual j heat load, the HDD for your area, the nature of your soil, the equipment used......
Your contractor knows the answers to these questions and has designed the system accordingly. For us to say "eh let's throw on an extra 13%" is based on little more than the notion that bigger is better. Curiously, "the smaller the better" theory, is more consistant with geo design.
Remember also that who ever designs the system is responsible for every facet of its performance. Don't inadvertantly take that responsibility off your installer's shoulders ("hey I did it like you wanted it....").
Good luck,
J
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