Paying 52,000 per year (awesome job btw!) makes this kind of a wash. You'll be done in ~ 27 months either way. If you plan to slow down your repayment I would probably get the fixed rate.
When you say a "huge chunk", does that mean you have more student loans than this? If that is the case, depending on the interest rates of your other debts, you might consider locking in a 4% rate for a longer term and throw the 52,000 per year at the other higher interest debt.
Great job maxing out 401k and IRA as well.