Looks nice, and like a perfect spot to put a tub. With a 4" thick slab, you should be fine. The tub will put a couple of tons of weight on the slab, but it will be spread out over a big footprint, so the psi won't be bad.
We put our tub on a pre-existing patio that had a slight slope (approx 1-2"/10 feet)as well. Just do a little planning regarding orientation of the tub, and you'll be fine. Our tub has deeper seats on one side of the tub, and with my wife and I both tall, we wanted as much depth as possible. We placed the deep seats on the downhill side, which leaves the shallower seats facing the yard. That works well, because those seats benefit from being up higher for the view. Although the water is closer to the cushions on the downhill side, no one but me notices. Another option we considered was placing the filter compartment on the downhill side to ensure that the pumps wouldn't cavitate, and give both deep and shallow seats the view.
If your slope is too much, someone here poured a shallow concrete shim on top of their existing pad, using a poly barrier to allow removal at a later point. That thread, with pics was on here about a year ago.