manchester city were beaten 4-2 by manchester united at old trafford .
defeat leaves city fourth in premier league , four points behind man united .
man city desperately need to strengthen an ageing squad in the summer .
and it remains to be see whether city will stick with manuel pellegrini .
but the biggest challenge could be keeping hold of sergio aguero .
argentine scored a brace against united to take his tally for city to 100 .