Best to figure out the train schedules and plan out your days to see when you would actually be realistically arriving and departing. Florence and CT works well but not so well with CT and Venice or CT and Rome so you have a long day for that leg of travel time which can be considered a good portion of a day lost.
Look at actual train times and then the next train in case you miss your first choice.
Likely when you map it all out you are going to lose more time than you originally thought and will decide on 3 days to make going out of the way to get there worth it.
If mapping out the times and routes does not sway your decision the other big factor is a combo of:
On your Florence days are you planning day trips to any of: Siena, San Gimignano (sp), Lucca, Pisa, Cortona, Volterra, Pienza, Montepulciano, Other Hill Towns, Day to tour the countryside, Day to go to Wine region, etc...
If you are planning more than one of these 10 options you are going to go through your 3 days quickly and may need the 4.
If you are staying put in Florence the whole time, 3 days would be plenty IMO and better to spend the 3 in CT rather than 4 in Florence in this case.
Likewise in CT, do you plan on hiking between the towns or taking the train or ferry. If into hiking you would need more time.
Do you want to see just the 5 villages or explore other areas further up the hillsides or nearby towns like Portovenere (on the main ferry route) as well? Do you want 1 full beach day in Monterosso or ok squeezing in beach time between hikes and other activities? Anything more time consuming like a day trip to Portofino?
After writing this all I think any planned day trips from Florence is really the biggest factor, once you do more research you wll likely conclude you really need 5 in Florence and 3 in CT.