y=u-x
y'=u'-1

u^2 (u'-1)=4
u'-1=4/u^2
u'=4/u^2 +1
u'=(u^2+4)/u^2
u^2/(u^2+4) du=dx
(1 -4/(u^2+4) ) du=dx
u -(4/2)arctan(u/2)=x+C
u-2arctan(u/2)=x+C
(x+y)-2arctan( (x+y)/2 )=x+C
y-2arctan( (x+y)/2 )=C ???