Firstly - I have a Betty Crocker super moist white cake mix which is 16.25 oz. Would putting all of the 16.25 oz cake mix in a 9 x 1.5 inch wilton round pan not be a good idea? If it's not a good idea, what is your suggestion? The recipe says it could make two 9-inch rounds. But I'm afraid that the cake will come out thin. That's why I'm thinking of putting all the cake mix in one pan.  Secondly - The recipe calls for 1 1/4 cups water, 1/3 cup vegetable oil, and 3 egg whites. Can I substitute these to 1 1/4 cups milk, 1/3 melted butter, and 4 egg whites? Are these the right amount for substitutions?  Thirdly - I want to bake four layers. Can I mix 4 cake mixes all together at once? If yes, then would I multiply 4 times of the substitution amount?  Fourthly - I want to avoid having a dome so I'm planning to use baking strips (made out of foil and paper towels). Should I bake the cake at 325 using home convection ovens? The instruction calls for 350 for nonstick pan (not sure if its for convection or conventional oven). And how many minutes should I bake it for?  This is a super long post. Sorry! I'm really looking forward to your reply. Thank you! :D 
Four thick layers is a very tall cake!  The affects not only the proportion of the size of the cake, but also the size of the slices.  Do you want slices that are 6-8 inches side to side?    Why not make three mixes and divide them among four pans.  That might work out okay.    When you are thinking of modifying a recipe, it's a good idea to do the basic recipe first so you know what you're dealing with.  As to heating cores -- you shouldn't need them for a cake less than 10 inches.  For large cakes, I put in several heating cores.    Heating strips are very helpful, but why the aluminum foil?  I just tied wet towel strips around the pan until I got my Wilton strips.    If you don't want "dome" on your cake you can always level it off using a knife or a "cake leveler" ("tortenschneider" in German!) to take off the dome.    If you want a butter cake why don't you get a scratch recipe and make that? Butter and oil cakes are often mixed differently. A white oil cake may have a better color than one made with butter (which, after all, is yellow.)  To get butter flavor you can find colorless butter flavoring for baking (Wilton, maybe?)  There is also colorless vanilla.  These are useful for white cakes (I speculate they exist to help people who want a really white wedding cake.)   Finally -- best of luck to you.  Don't be "afraid" of what will happen; think of yourself as curious about what will happen.  We call it "gaining experience." 