Fully deductible interest rate of 4.5%. Why pay that off early???
My rational is mostly behavioral. It is a small small debt amount, the return is guaranteed, and the action creates the right behavioral precedent.
When ready, they should also invest some amount for the long term, but not until they are ready. By asking the question, the OP presents the impression of a fairly new, young, and less educated investor. IMHO, no one should invest in stocks without 1) knowing the risks fully, and 2) forming an asset allocation plan with the idea thar that part of assets will be held for at least a 5 years + timeline. An IRA or 401k generally enforces the long term.
So my interpretation is that this person would be better off killing the loan first, as habits trump what might be a slightly higher expected return (but not a guaranteed return, risk adjusted). They are already maxing the ROTH IRA, so getting some long term stock exposure, i assume in INDEX funds. Once loan payments are gone, more to invest.