The problem with mounting a box on the outside of the siding, and depending what kind of fixtures you use, they might not seal around that box, letting water in. Also looking bad. If its a flush mount fixture, they are meant to be tight up against the finished surface. If its a fixture that is made to go on a weatherproof box, then that will be ok. Don't know your situation, Put the siding up, drill your 4" holes, and as long as you can get to it inside, fit your box to the holes. You can use blocking or there is a variety of brackets on the market to support that box. Like I said, don't know your situation of how far the studs are apart or anything. Maybe a picture or two. Main thing, if the fixture is meant to install up to the finish surface, I defiantly would recess the boxes