Four months is really not THAT long for a bike to sit. If the battery was not on a charger, you probably want to at least trickle charge it for a day or so. If it won't charge up, yes you'll have to replace it. The fluids should be fine, if they were ok beforehand. I wouldn't change the oil if it was changed 400 miles ago, unless you are really anal about it. I would look in the airbox to make sure no critters have taken up residence in there, pump the forks a few times to look for seals leaking.
Nothing wrong with adding a bit of Sta-bil to the fuel. I use Sta-bil, but usually -before- I put something in storage. If you want to put something in the fuel to help clean the injector etc, Techron FI cleaner works very well. Put in fresh fuel and a few good dollops of the Techron, and go riding!