I would suggest knowing a couple of backup options. There are 3 motels along St. Jacques street.
You can call and find out from them what their prices are. Just call reception and ask if they provide shorter bookings. Many places offer either a 3 or 4 hr booking option.
I don't provide incall, but I did call like 8 places I found I frequented about a year or 2 ago to do just that, and made a list on my phone that I was able to provide people when they asked me where they could book. having options is important in case the one you want is fully booked.
Setting up an appointment, unless you're plan is calling an agency and just asking for who's available, might be a good option before booking the room, (call to see availability that day first I guess) as you don't want your room rental to time out, before you find someone to come see you.
Call the locations to research their price direct through them first. Then you can check online. See what prices better suit you. There's a chance there's a markup in price to book through a service site, or see if there's a special, but something tells me, the more businesses between you and the room, the more businesses looking to get a piece of the pie from the cost, raising the cost of the 4 hr room booking. (And data tracking for price gouging, not all technology is a benifit.)