Do the Canadian side. Falls have a tunnel that isn’t horrible, but not exciting. Maid of the Mist is a staple. There are a lot of “museums” and attractions in “town”. Like a wax museum, for example. DO NOT waste money on the Guinness World Record place. If I recall right it was mostly a bunch of plaques on the walls. So the town looks a bit like a tiny Times Square the way the buildings are all done up to attract customers, and everything costs exactly twice what the experience is worth.
I saw a show that featured a speed boat thingy ride - that looked exciting. Which is why I didn’t do it. :)
Burger King charges about double in US currency for the same meal as in the US. Something about exchange fees. And the staff were haters.
Nearby is the wine trail, which I didn’t do. We only spent one full day in the Falls because that will provide ample time to do the Falls and the town.
I have no idea how one might occupy oneself for four days there. On the way out we went in search of the original Buffalo wings restaurant. A bit tough to find, it is the the old section of Buffalo. I don’t eat hot wings, but my husband loves them. He said those were good, but not spectacular.
The American side of the falls can be a bit dicey. Nearby neighborhoods are bad and very rundown.
If you’re there for four days, you can definitely squeeze in a day trip to Toronto.