With the recent ‘domestic’ terror arrests and attacks, a beheading in Oklahoma of all places, I’m actually surprised that it took this long.

Agreed, the same intel, the same guys, just staged for work.
Do I suspect that they have more than whispering of things to come? Of course.
At the same time, the general public panicking could be even more destructive than an attack. Living in fear is what the terrs want, don’t give it to them.

Keep your gear ready, keep your CCW handy, keep your eyes open.

And why then 4th specifically?
Its historical significance, people will be relaxed and complacent, and anything that happens will be masked by the fireworks.