Every additional piece of glass adds one more layer of noise between the lens and the target, so stacking them is not an optimal way to get a great image. That said, sure you can do it.
First things first, if you're going to shoot with an ND, then take the UV filter off first. More times than not the UV is on there for protective purposes only, and you'll have an ND (or two) on there, so lose the UV. Also remember, when you stack filters you are adding barrel length, which can get in your way on a wide angle. I stacked 2 on a 16-35mm and lost the corners.
Second, I've yet to find an adjustable ND filter that will get you past 6 stops before you get unevenness across the frame. I bought and returned 2 different adjustable ND's and both exhibited the same light & dark areas in the dark 1/3 of the swing. You're also dealing with 2 pieces of glass in these, so even at low values you have 1 more element that you'd otherwise use. I hear there are good ones, but they seem to be prohibitively expensive.
As for stacking, if you've got good filters, go ahead - but don't do more than 2. Also, figure out what you want to achieve with a fixed ND (vs. a gradient, not variable) and buy appropriately - a lesson I learned the hard way. An ND 4 will give you 2 stops, which is something you can get with exposure compensation if you think about it. I found a good deal on and ND 2, 4 & 8 set of Hoyas and bought them figuring, "Heck, if I stack the 4 and 8 how much more would I want?!" The answer is, probably more, especially on a sunny day. My recommendation would be to return the 4 if you can and get at least an 8 (3 stop) to begin with. But if what you're looking for is something that will smooth out running water in the sun, you're going to have to go with a 9 or 10 stop filter, so you might as well start there. I carry an ND 8 and ND 400, figuring that's 12 stops stacked if I ever wanted to do that, which is an awful lot.
One more tip, buy them in the largest size you're likely to use (for me that's 77mm) and then get a set of extender rings for your other filter sizes. I have a good 67-to-77 ring for my 28mm & 85mm 1.8's and a 72-to-77 for my 28-85mm zoom. I also got a set of stacked extenders that will allow me to ramp down to just about anything - just be careful with them and never screw them together even mildly tightly.
