>>65This.
Specific stocks can drop 100% in a year, stock indexes can drop 50%, bond indexes 20%, etc. All the positive returns you hear about are after things average out over several years. The only guaranteed returns are slow (CDs, T-bills, savings accounts).
If I were you, I'd take the $400 extra and add to the $4500 in the bank until I had $1200*6=$7200 for an emergency fund. Maybe build it to 9 or 12 months if you're a paranoid parrot. Beyond that, paying off your student loans would be guaranteed return. What's their interest rate?