Flamestrike
Legend
To do something like this, you need waves of about 1300 XP each (according to the dmg for 4th level charecters, 250 XP per charecter for medium, 375 for hard, I am doing a rough average of 325 times 4)
skeletons are 50 each, scarecrows 200, vultures 10 (giant vultures are 200).
OK, it gets a little harder. When you have multiple opponents, you are supposed to adjust HP. But if your party can hit flying targets and multiple enemies--through area spells or great weapon fighting or two weapon fighting or something--I would not adjust that much. And you won't get much of a wave. Lets say double it.
If you have 5 vultures with skeletons and two scarecrows before adjusting that is 700 which becomes 1400. You can drop some to make this a bit easier, though this actually feels about right, especially if there is a delay before the scarecrows make contact. You will probably want to mix things up (consider a few of the giant vultures at some point), but this should work as a sort of benchmark.
Actually the 'XP per adventuring day' chart mentions both adjusted XP and earnt XP.
Based off the numbers, I reckon its more likely 'earnt' XP (i.e. XP before adjusting for difficulty).