And you can use fertidust until you have three branches, but you must just use pet/water to get the 4th branch. If you use fertidust to get the 4th branch it will not count. :)
btw, since i've had this discussion with a gazillion people...when a wood tree has 4 branches, you get 2 chops before it goes to 3 (first chop stays at 4 branches, 2nd chop takes it to 3). If you have a 4-branch tree that has already been chopped once, no amount of petting/watering/dusting is gonna get another chop outta it. To go back to a full grown 4 branch 2 chop tree, you have to take it to 3 branches and then dust or pet/water it back to 4 branches.
so stop glaring at me for not petting or telling me to dust when there's still 4 branches everyone :P