Thats just my opinion. Even a guinea pig would have to be taken care of mainly by you, as a 4 year old is still very young.
I agree with letting kids touch and help take care of animals at an early age.
** BUT always supervised ! They can get bite, they can hurt the animal, by dropping them or squeezing them to hard, so on and so forth.
But it is also fun to let them name their new friend, help to get water, and to hand feed veggies and treats. Even to help clean, by getting the things you will need for the job.
It can teach them so much. I had a guinea pig very young, and loved them. They squeal when hungry and it was neat hearing them 
Where a bunny is a wonderful pet but would be very hard for a 4 year old to hold, a child that young could get really kicked and scratched up trying to pick a bunny up wrong.
Maybe you can get a bunny a few years down the road. After you see how interested and good your child is with a smaller critter.
If you would like to ask more guinea pig questions try the guinea pig forum, they will be glad to help.
And remember, piggies can be hurt, so you will have to be there, also they can nibble a bit, not hard, usually they just want something more to eat 
Also they like company, hehehehe, so don't really plan on one, if you are going to do it get 2 piggies. 
Good luck, wish you luck in your quest for a pet!
Let us know how you make out!
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