If you have time on your hands I suggest you use Route 2 instead of 90 when you enter MA. Use 90 till the exit to Lenox eat in lunch in Lenox and stop at the Norman Rockwell Museum. If you want to spend the whole day there you can drive up to the top of Mount Greylock and see about 70 miles on a clear day. Spend the night in Williamstown or North Adams. The next day head east on Route 2. Make sure you stop for lunch near the Quabbin Reservoir, just to the south of Orange, MA. There are about 50 state parks in western mass, looking on google maps at the parks is amazing. It is management area, state forest, or conservation area in most of central mass between Gardner and Amherst. Try to get a picnic lunch and take it to Quabbin Park, you can hike or fish the lake from there. You will be near 90 by that time. From there it is about an hour and a half drive to Boston. Great that you have four days because most people rush in on 90, the least scenic highway in the most scenic area in Massachusetts. I do not know as much about New York but I have driven western Mass multiple times for the weekend. Have fun on the trip.

Just east of Rochester NY you could visit the Finger lakes, for scenery and also many wineries.

you could make a stop in Cooperstown for the Baseball Hall of Fame and the Fenimore art museum. Its a cute town too.

Howe's Cavern is just off i90 in New York, and is worth a quick stop.

A very interesting small museum is the Arkell Art Museum in Canojoharie. They have a small but lovely collection, as well as some historic displays about the history of Beechnut food which was founded in that area. (Bartlett Arkell was the first president of Beech-nut and collected art, in part to display in his factories). http://www.arkellmuseum.org/

If you like Hudson River art (those great landscapes of Church and Bierstadt), you might want to stop to see Olana or the Thomas Cole house, both of which are not far off i90 in New york, close to the Massachusetts border.

I definitely agree that the Berkshires is worth a stop, and could easily fill a couple of days. You will find several very good trip reports about the Berkshires if you do a search. the Rockwell Museum has a very good special exhibit of Howard Pyle through October. I saw it and liked it very much.

If it interests you, you could stop at Sturbridge Village. Generally, i think of this as a thing to do with kids, but its kind of fun even for adults.

There are probably more things I can think of. There's really a surprising amount of things right along i-90, and you could stretch it even more with a side trip to Lake George or Lake Champlain if you wanted to.

Also in Western Massachusetts you can visit Hancock Shaker Village. http://hancockshakervillage.org/

And stop at Shelburne Falls to see the Bridge of Flowers. http://www.bridgeofflowersmass.org/

Finger Lakes is good for at least two days, easy to drive, wineries, lake cruises. Corning/Ithaca, Letchworth State Park, Watkins Glen State Park, have fun