Fourthly rhymes with:
bimonthly, blithely, deathly, earthly, keathley, keithley, keithly, monthly, unearthly, worthley
Fourthly sounds like:
fairytale, faretlow, ferdlow, feretlow, fertile, fertlow, foretell, fredell, fredella, freedle, freidel, fretwell, friddle, fridley, friedel, friedl, friedley, friedli, friedly