Travel on July 4th is likely only "nightmarish" if you are driving. Air/rail will be standard crowded high-season in the northeast. Don't give it a second thought.
While Amtrak is a good alternative to air between WAS-NYC or NYC-BOS, it is a distinctly longer trip WAS-BOS. For the former hops, it is debatable whether air or rail is faster city center to city center, but rail can be more pleasant and more relaxed.
For WAS-BOS, you have multiple air options for what, depending on time of day can be just under or over 60 minutes in the air vs. 8 hours on the train. Typically, B6 will have the best advance fares, although it is always worth doing a search for your specific date & time.