∵-3/7a^(2+m) b^(2n-1)是四次单项式,
∴2+m+2n-1=4,
∴m+2n=3,
m²n-m²-m（4n+mn）-（-2n）²
=m²n-m²-4mn+m²n-4n²
=-m²-4mn-4n²
=-(m+2n)²
=-3²
=-9