Buongiorno,
I suggest that you take your 4 days and divide them into
visit Florence for one full day (make Florence your base because it is easier to get around from here),
one day in Siena via public transport and explore the city
one day a tour into Chianti/San Gimignano
travel to Orvieto, sleep here the night so you can enjoy the town at night
and catch an early morning train into Naples (anywhere from a 2 to 4.5 hours train trip)
Buon Viaggio
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