What do you mean, unrealistic?
What part? Staying in CT for two nights? Or going to CT altogether?
I think you are trying to do too much and too little time. Since you haven't been there before, it's hard to get a feel for distances and logistics.
I think most of us want to "do it all". We have done trips where we tried to do too much. We also have this underlying fear that we'll never get a chance to go again, which leads to, in the words of Elvis, a "It's Now or Never" way of thought.
You have made three choices, any one of which would work. If your heart is set on CT, then you need to do things to make sure you have the best possible visit. That means staying there at least two nights. As Kathy and others have mentioned, you need to be able to get up early to be there before the cruise ship crowds and stay up late to enjoy it when they have all left.
There's a couple of problems IMO. That leaves you with a block of time during the day when you'll be trying to avoid the crowds. Precious time will be used while you are essentially in idle. The other problem is that 4 full days in CT is more than enough, but you really don't have enough time to adequately devote to other areas. That's why I think Tuscany or Florence are better options.
Tuscany would let you enjoy another aspect of Italy that is different from the cities. Since Siena is a bigger town, I would lean towards Volterra, which is a lovely hill town. To make the most of your stay, you'd need a car. That would allow you to explore the countryside. San Gimignano is only 25-30 minutes away and you could even do a day trip to Siena if so inclined. You could even consider staying at an agriturismo outside Siena. There are some agriturismos that are wineries which would allow you to indulge without worrying about drinking and driving. Of course, if you stay in Volterra, there's plenty of opportunities like here to sample wines.
I'm partial to Florence, which will always hold a special place in my hear. My wife and I met on a school trip way back in 1966 and shared our first kiss in Florence on August 10. There's a myriad of reasons it would be a great choice for you, but since you'll be seeing Rome and Venice, a taste of the countryside might give you a more rounded view of Italy.
BTW, when are you going? Time of year will be a factor for CT.
Whatever you decide, I hope you have a wonderful trip and it is just the first of many trips to Europe!