Montecatini is it worth four nights

In Montecatini itself, no. There is a funicular to Montecatini Alto,but that's only worth a half day. The spas are very expensive. But it's on the rail line, so you could go from there to eg Lucca.

But you haven't said what the rest of your trip looks like, so hard to make suggestions of what to do there without more information. Florence, Siena, Pisa etc?

Confused when you ask if there is enough to do in Tuscany for 4 nights. I'm assuming you meant Montecatini, per the heading?

We spent 6 nights in Montecatini. We day tripped from Portofino to Siena, including Pisa, San Gimingnano, Monterigioni, Lucca, Florence, and lots of smaller places. We did it with rental car mostly but used rail to and from Florence which is a very easy trip. We enjoyed Montecatini Alto for dinner, cute place. Plus, Montecatini itself was a very relaxing base, good restaurants and piazza with great coffee. We enjoyed our stay there.

Tuscany is worth more than 4 nights, as you will see when you get there.

We spent 7 nights in Montecatini and did day trips to many many Tuscan towns. I was on a bus tour so we did the day trips by bus but with a car and the trains so many great places are easily reachable. We went to Florence, Siena, Lucca, Pisa, Cortona, Arezzo, Voltera, San Gimignano, Prato, Pistoia, and of course Montecatini Alto. Some of these you can do two or more in one longish day. I found it a great base for touring the area and Montecatini itself was a very nice relaxing place to spend the nights. Loved staying in one hotel for the whole time. There are enough good restaurants for dinners and it's a lovely place to walk around in the evening. We spent one evening in Alto for dinner and a stroll. I have to give a caveat here - my stay was quite some time ago so I can't vouch for what it's like now as far as relaxing vs bustling and crowded, or for restaurants, etc.