spaying older dogs

Question
My dogs if around 4 years old is it still good to have her spayed?

Answer
Yes.  The fewer seasons a dog goes through, the less likely she is to develop cancer.  It also eliminate the mess, the hassle, the risks of unplanned litters, infections, getting lose and getting killed.  There is no good reason to keep an unspayed female other than as part of a serious breeding program.  Even breeding stock usually is spayed once they are too old for breeding, usually no more than 5 litters.  

See http://www.avma.org/animal_health/brochures/spay_neuter/spay_neuter_brochure.asp