I didn't have a hole saw that size, but I had a 3 incher and a jig saw and figured I was all set. This would have been fine if the exterior side was not pinned right underneath a friggen deck. I drilled my 3 inch hole figuring I would just simply "fine tune" on the outside. After my "starter" hole was finished I whistled my way outside and as I climbed up the ladder I realized I had no clearance for my jig saw due to the lovely deck at the top of my head. I was now in between 2 joists with very limited movement. My whistling quickly transformed into some good ole fashioned cursing. I wasn't even sure I could use a 4 inch hole saw now since there was nothing for it to grab onto since I just drilled a 3 inch hole! Man what a chump. I had no choice but to get a 4 1/8" hole saw and give it a shot. I actually nailed a piece of scrap wood OVER the 3" hole to give my new hole saw something to grab and pilot against. For those that have used a hole saw before you know how crazy they can whip out when it has nothing to grab onto. This along with being on a ladder multiplied by limited mobility thanks to a deck above = Total Chump. After some heavy cursing and some determination my tactic seemed to work! I was now on my way to finally just cutting a damn 4 inch hole to simply put a stupid vent thru. I got about 75% of the way thru and my #$%&$'ing dremel broke now. My new hole saw was rendered useless and was just "freewheeling" with nothing to grip on. I used the wrong dremel I learned. The dremel I was using was meant for my smaller hole saws for door knobs and stuff. I could not believe how long a 15 minute task turned into. I NOW was forced to use a sawzall and finish the rest of the cut out in an extrememely brutal manner. After this total gong show I stuffed up the hole and called it a day. Lesson learned, always use the right tool or else your just wasting your time. I now boast a rather fancy collection of hole saws and other various power cutting tools.