I'm not understanding why my New York number is $28,500 when I've only been here for 4 months.

Did you move to NY and become a legal NY resident, even for just 4 months of 2016?

If so, you may see your entire 2016 income listed for NY, even though 8 months worth was earned while you were a resident of another state.

Even if you were a resident of New York for 1 day, NY uses your entire AGI to calculate your tax on NY income. It isn't, however, as bad as it appears. They calculate what your New York tax would have been if you earned everything in NY, but then prorate that tax based on how much of your income was earned while a NY resident. 

For example, if the NY tax on your entire 2015 income was $10,000 but only 30% was earned as a NY resident, your NY tax would be 30% of $10,000 = 3,000.

Why do they do this? So they can tax your NY income at the highest possible tax rate, based on your entire AGI.