Yep, Boulder Drop is probably always class IV, but in low summer flows its one of those that people argue about. It's a fine piece of whitewater! You can make it class IIIish, or there are class V lines to be had. Best way to spend your day is hanging out there and lapping it. It's the best training ground you'll find.
My favorite, enter far left and find the little rock boof/smear move. Probably a good idea to follow someone your first time on that line though...easy to get lost in the rocks if you know what I mean.
Seriously, go there. There are hundreds of different lines in that rapid.