I've owned guinea pigs, gerbils and hamsters. Hamsters win, hands -- or, paws -- down.
- They're smaller; thus, they don't make as much poo, which means the cage needs cleaning about every week and a half instead of twice a week. The smaller cages are quicker to clean, and you don't need as much bedding (buy CleanFresh -- nothing else, ever).
- If you go out of town, it's super-easy to take the smaller cage to someone else's house to be babysat.
- They don't pee on you as often. We always held the guinea pigs in a hand towel. If they do pee on you, the quantity is smaller.
- They're more active and enjoy climbing through tunnels, so you can always "upgrade" their cage. You can save up wrapping paper tubes and oatmeal cans and make homemade obstacle courses for them.
- They don't squeal every time the refrigerator door opens, though they are just as greedy as a guinea pig. Ours hangs on the side of the cage, if we eat nearby. They love fresh vegetables as well as store-bought hamster treats.
- Hamsters are territorial and much prefer to live alone; thus, you're free from guilt if you don't want to undertake two critters. If you buy two, even as small hams, you're going to have epic battles on your hands.
- Hamsters are considerably slower-moving than gerbils. Probably a good thing for a four-year old.
Negatives:
- Being smaller, if they escape from the cage, they're harder to catch. We've owned two wire cages, which have proven escape-proof. We had a plastic Obo (Obo?) cage once, and the little fellow could scoot right out. To minimize this problem, buy only the larger teddy-bear varieties (sold as Honey Bears, Panda Bears, Black Bears, etc.). The dwarfs can really scoot, which will especially be an issue with a four-year old who might drop them occasionally.