Do not ever say anything that even remotely suggests a possibility (no matter how distant) of your child getting a hamster. If you do, said child will spend the next hour looking at hamster cages, toys, equipment, etc when you really just wanted to run in and get some dog food.
When your husband comes home from work, your child will then run up to him and excitedly declare that Mommy is getting him a hamster when he turns 5. (Oh, and don't try to put off the hamster excitement by saying maybe for your birthday if you're really responsible and do tuvwxyz, etc.--listing all the necessary requirements for responsible pet ownership will not make an impression. Even if you start making stuff up, the child will remain undeterred.) DH will be very upset and you will end up fighting over a hamster that you don't even want while your son dances in the background chanting that he will clean the poop every day, even if he has to touch it and get hamster pee/poop all over himself. (attempting to gross out a 4yo boy with the nitty gritty details of hamster care is futile.)